After 2007 (efi) are all the sportster fuel tanks interchangeable between models and if so what model has the biggest tank from the factory and size , same as the seats , are they all inter-changeable between models .

 

yes..

 

Customs have 4.5 gallon tanks

 

Some Roadster models/years have 4.5 glln tanks, too.
"Low" models, as well.
Some seats for models with a 3.3 glln tank are longer than factory seats for the 4.5 tank. Along with the length issue, is the placement of the ECM. '07-'09, it's under the seat. Factory saddles have a pocket for the ECM. 2010 up, it's not under the seat.