Panic,
Tappet diameter is determined by the maximum valve lift velocity, if the velocity is too high, the cam will overrun the tappet face. The illustrations posted by Tim and Greg show the lobe "elevation view" of the situation, but the velocity portion is best considered as the lobe reaching in and out of the page. Think of what the cam and tappet look like if you could view them along the camshaft instead of beside it. Most automotive applications have settled on a few 'standard' diameters since most engines performed well with the velocities those allowed.
The best reference for this is a book by an oldtimer named Mike Turkish. He worked for Eaton Corporation's Valve and Lifter division back in the 1940's 1950's. I have a copy of his book somewhere deep in my files. If I can lay hands on it, I'll post some background.
Interesting topic.
Last edited by BDammers47; 11/20/2015 9:04 PM.
