The B+W Push-On Lens Cap #300 model is their 'standard' unit, and has a slightly convex front surface to prevent the inside of the cap from touching the glass of your lens when it is tucked away in a bag. The sides are their 'standard' depth, to fit on normal lens barrels or standard-style filters, and this particular unit has a 51mm inner diameter.
B+W also makes a 'shallow' sides model, the #305 model, to fit on slim filters, and a more common snap-on cap.
It isn't an exact science due to all the variables involved, but generally a push-on cap that is 2-3mm larger than the lens' filter size fits just right.