The curtain/mosquito net is mounted from the inside and should be larger than the hatch. This means there should be an overlap for the curtain/mosquito net. This also means the curtain/mosquito net also fits smaller hatch. Just measure the inside of the opening of the hatch. Often it's referred as the "cut out" size by the hatch manufacturing companies. You are supposed to measure side to side not across.
The name of a hatch is often referred to the outside measure of a hatch. For example a standard Lewmar 60 has a hatch has a cut out size of 518x518 mm. A general rule is that a standard hatch is ~50x50 cm and it's built so a normal person can climb out of the hatch. Boat owners with large hatches often know that the hatch is large.