Well, most shoes will be compatible with 2 hole (mounting screw hole) cleats or 3 hole cleats. Two hole cleats small. They tend to be in the 1inch wide x 3/4 in long size. Pretty small.
Road cleats are almost all 3 hole cleats. They tend to have a large platform of 2 to 3 inches across.
Mountain bike shoes are almost all 2 hole cleat compatible. Road shoes will be compatible with 3 hole cleats, and sometimes with 2 hole cleats also, on the same shoe.
The best way to find the right size is to go to a shop and try them on. Don't buy online unless you know how the shoe fits. They will be a snugger fit than your everyday walking or running shoes.
Tell us what cleats you have.