Measure the area you have for a fuel tank. Look at the DuBro and Sullivan web sites for the actual sizes of tanks that are available. Allow a little extra for foam padding.

THAT will be the largest tank that will fit.

3 minutes on a tankfull? Either you're not getting all the fuel out, it's a 2 ounce tank in there, or the Saito is running way too rich.

(Probably the last one is right)