Size is the biggest determination of what you can do with any given end mill. Large ones excel at grinding through a lot of material at once, but you don’t get a lot of detail out of your parts. With CNC milling, the radius of your end mill is the radius of any internal corner, so you will almost never have a perfectly square corner on the inside of a milled object. Smaller and smaller end mills can be used for each pass to clean up an edge and get the part to the final dimension and shape.
http://www.douruytool.com/cnc-end-mills/