Place to find and convert bike size from brand to brand?

The best way is to measure you actual inseam from your crotch to the floor. You want a frame with the stand over height of 1-2 inches less then that. Another thing to check is the top tube length on a bike that fits and compare that to the one you want to buy. Manufacturers have difference stand over heights for the same frame size. The stand over height is more accurate.