A: There are several shaft sizes used with tuning machines, ranging from 5/16″ to 11/32″ (many vintage-style tuners) to 13/32″ (10mm; many modern tuners); other sizes were also used.
The easiest solution may be to remove one of the current tuners, and measure the diameter of the peghead hole it was in. Then check out the specs for the tuner you want to use to determine if the size is correct.
If the current hole is too small for the tuner you want to use, you can bore out the peghead (though, of course, this is irreversible, be sure before you proceed!) If the current hole is too large for the tuner you want to use, conversion bushings are available to correct the problem.
The other question will be whether the mounting screw holes in the tuning machines line up correctly with the holes used by the current tuning machines on your guitar…