Dell oem psu for 7090 goes up to 500w. Y7R0X 500W Chicony99TPH 500W Liteon5K7J8 500W Delta dimensions 85x65x225 mm vendor recommendation of 650w could be exaggerated to cover all possible config and may not apply to prebuilt pc. if you still want 650w psu. can try non-Dell thin form factor from Silverstone TX700 gold.  you would need a 24 to 6 pin adapter too.  
I'm replacing an existing PC with an Optiplex 7090 MT and found that the existing PC has a DAQ card that the vendor recommends a power supply at least 650W. The current power supply is rated at 240W or 260W. Does anyone know if there are any upgrade options available that will fit the smaller tower? Thanks.
Power supply upgrade options for Optiplex 7090 MT