The geographical extent of Tomislav's kingdom is not fully known.
John the Deacon, whose chronicle is a primary source for the history of Slavic peoples in Dalmatia during the 9th and 10th century, wrote that in 912 a Venetian ambassador, returning from Bulgaria, passed through Croatian territory before reaching the land of Zahumlje under Duke Michael, which suggests that Tomislav's Croatia bordered Bulgaria, then under the rule of Simeon I. British writer Marcus Tanner suggested that it covered most of modern Croatia, Bosnia and Herzegovina, and the coastline of Montenegro.
However, Roger Lampe argued that the state did not go as far south as Dubrovnik and that Istria was not included.
Many Croatian scholars argued that the kingdom covered the whole region south of the Drava river to the Drina and Neretva rivers north of Dubrovnik.

Nevertheless, Croatian historian Nada Klaić disputed the eastward (i.e., Bosnian) extension of Tomislav's kingdom in her 1972 book.

Josip Lučić and Franjo Šanjek's 1993 Hrvatski povijesni zemljovid (Croatian historical map) provided an extended depiction of Tomislav's kingdom.
Lučić was a known historical geographer from the Faculty of Philosophy in Zagreb who authored numerous maps in Croatian history books.
Ivo Goldstein in turn argued in his 1995 book Hrvatski rani srednji vijek that Tomislav made no crucial expansion deeper into inner Bosnia and that he incorporated only parts of Pannonia to the Kingdom of Croatia, not the whole area between Drava and Sava, which was according to him Terra nullius.
Neven Budak is in full agreement with Goldstein regarding the status of the Drava-Sava area, arguing that the northern Croatian border most likely passed through the wider Sisak area.
Dominant modern university history textbooks in Croatia such as Tomislav Raukar's Hrvatsko srednjovjekovlje (1997) states that during Tomislav's rule his kingdom covered between 60% to 80% of contemporary Bosnia and Herzegovina.
Franjo Šanjek also edited a major work by sixteen authors on the medieval Croatian state,also used as a university textbook, which includes this view.

John Van Antwerp Fine stated that Tomislav's northern border was the Drava River.
South of it he held "modern Croatia, Slavonia, northern and western Bosnia, and the territory along the Dalmatian coast from what is now Rijeka to at least the mouth of the Cetina River (excluding the scattered Byzantine towns)".
In his 2006 book, Fine criticized the relationship between Tomislav's territory and modern nationalist sentiment in Croatia, saying that 10th-century sources are unreliable and "roughly a third" of Croatia's perceived eastern land is "entirely speculation".
Fine stated, "It is possible that Croatia really did have some of it, but Bulgaria may have had some of it; early Serb entities may have had some of it, not to speak of various župans and other local Slavic lords who in any serious way answered to no one.
If the last supposition is true (to any degree), then parts of this territory would not have been held by any 'state.'"
While acknowledging the possibility of Croatia having held all the depicted territory and more, Fine stated that whoever controlled the eastern land depicted in Tomislav's kingdom is unknown and should be marked as terra incognita in maps.
He criticised Lučić and Šanjek's delineation of Tomislav's eastern border as "nationalist map-making" and distorting the perceptions of children on their nation's history in a way that promotes interpreting later events as territorial loss and fragmentation.
The geographical extent of Tomislav's kingdom is not fully known. John the Deacon, whose chronicle is a primary source for the history of Slavic peoples in Dalmatia during the 9th and 10th century, wrote that in 912 a Venetian ambassador, returning from Bulgaria, passed through Croatian territory before reaching the land of Zahumlje under Duke Michael, which suggests that Tomislav's Croatia bordered Bulgaria, then under the rule of Simeon I. British writer Marcus Tanner suggested that it covered most of modern Croatia, Bosnia and Herzegovina, and the coastline of Montenegro. However, Roger Lampe argued that the state did not go as far south as Dubrovnik and that Istria was not included. Many Croatian scholars argued that the kingdom covered the whole region south of the Drava river to the Drina and Neretva rivers north of Dubrovnik.
Nevertheless, Croatian historian Nada Klaić disputed the eastward (i.e., Bosnian) extension of Tomislav's kingdom in her 1972 book.
Josip Lučić and Franjo Šanjek's 1993 Hrvatski povijesni zemljovid (Croatian historical map) provided an extended depiction of Tomislav's kingdom. Lučić was a known historical geographer from the Faculty of Philosophy in Zagreb who authored numerous maps in Croatian history books. Ivo Goldstein in turn argued in his 1995 book Hrvatski rani srednji vijek that Tomislav made no crucial expansion deeper into inner Bosnia and that he incorporated only parts of Pannonia to the Kingdom of Croatia, not the whole area between Drava and Sava, which was according to him Terra nullius. Neven Budak is in full agreement with Goldstein regarding the status of the Drava-Sava area, arguing that the northern Croatian border most likely passed through the wider Sisak area. Dominant modern university history textbooks in Croatia such as Tomislav Raukar's Hrvatsko srednjovjekovlje (1997) states that during Tomislav's rule his kingdom covered between 60% to 80% of contemporary Bosnia and Herzegovina. Franjo Šanjek also edited a major work by sixteen authors on the medieval Croatian state,also used as a university textbook, which includes this view.
John Van Antwerp Fine stated that Tomislav's northern border was the Drava River. South of it he held "modern Croatia, Slavonia, northern and western Bosnia, and the territory along the Dalmatian coast from what is now Rijeka to at least the mouth of the Cetina River (excluding the scattered Byzantine towns)". In his 2006 book, Fine criticized the relationship between Tomislav's territory and modern nationalist sentiment in Croatia, saying that 10th-century sources are unreliable and "roughly a third" of Croatia's perceived eastern land is "entirely speculation". Fine stated, "It is possible that Croatia really did have some of it, but Bulgaria may have had some of it; early Serb entities may have had some of it, not to speak of various župans and other local Slavic lords who in any serious way answered to no one. If the last supposition is true (to any degree), then parts of this territory would not have been held by any 'state.'" While acknowledging the possibility of Croatia having held all the depicted territory and more, Fine stated that whoever controlled the eastern land depicted in Tomislav's kingdom is unknown and should be marked as terra incognita in maps. He criticised Lučić and Šanjek's delineation of Tomislav's eastern border as "nationalist map-making" and distorting the perceptions of children on their nation's history in a way that promotes interpreting later events as territorial loss and fragmentation.
Tomislav of Croatia