We had a 5 gallon aluminum tank on the race quad, pits were planned every 50 miles. Avatar pic is the Zilla at the '93 Baja 1000.
Thats a good safe number Dez, Stock tank hold close to 3.50 gals, a 5 gal tank would be nice! every engine is different depending on all kinds of variables from carb size, compression, porting , size of rider, flat ground vs hills, rpm, open air box, closed air box and so on.