" 

 :The star atlas: 

 ::*Will Tirion, ""Sky Atlas 2000.00"", Sky Publishing Corp., 1981, ISBN 0-933346-33-6. 

 :has the great orion nebula between about: 5h 33m to 5h 37m; -5° 0' to -6° 5'. That's roughly 65'(RA) by 60'(Dec). If you include the northern cloud (1977), the Dec. goes from -4° 35' to -6° 5'. That would give roughly 60'(RA) by 85'(Dec.). For just NGC1976 (M42) I get 5h33.5m to 5h36.3m; -5°12' to -5°50', for roughly 42' by 38' (round to 45' by 40'). 
 :On the other hand: 

 ::*Roger W. Sinnott and Michael A.C. Perryman, 1997 ""Millenium Star Atlas"", Vol. 1, chart 278, ISBN 0-933346-82-4. 

 :has the nebulae (NGC 1977, 1982, 1976 and 1980) between about 5h 34m to 5h 37m RA; -4° 50' to -5° 55'. That's roughly 45'(RA) by 65'. Just NGC 1977 is the same in RA, but Dec. -5°15' to -5°40', for 45'(RA) by 35'(Dec.) 
 :Personally I'd go with Tirion as it's more inclusive. If somebody has access to: 

 ::*Hirshfeld and Sinnott, 1985, ""Sky Catalogue 2000.0"", Vol. 2. 

 :That might be a better reference. Or maybe there's something in ""Burnham's Celestial Handbook"" that will serve? Anyway it looks like the listed dimensions are for the entire cloud, rather than just the emission nebula NGC 1976. —  "