tire width

 How can you determine the maximum width of tire that will fit a rim?
For a 125TT or CR150 or the like. How can you determine the fattest
tire that will fit the rims?
Example: the Yamaha 125TT has a 70/100-19 front and 90/100-16 rear.
If I wanted a fatter tire fore and aft what should I shop for?

Ron