Sometimes you already have an idea for the size you are looking for. This page should hopefully help you to find what you need. You can only search by width or depth, not both together, so you may find one thats right for you in either one. Bear in mind that you can have a cabin built to whatever size you like, and you can find out more information about that here.
Remember, when measuring to see how much room you have, don’t forget that you will need to get behind the cabin to maintain it, so don’t put it too close to the fence!