Cat Flaps, which size?

I have just linked my shed to the rabbit run and now need a cat flap so the rabbit can use it. I was thinking of making my own, but it would be better if I just bought one ready made as it would be easier. The main thing is that it needs to be weatherproof,draftproof and lockable. I am not sure which size is best to go for. The normal size ones seem quite small to me. Our rabbit is a 3 month old Rex and can get through small spaces now, but i'm not sure how much bigger he will be when he's fully grown. The cutout in our shed is 23.5cm x 27cm. I have seen a 'Catmate Large' which is the largest one I could find that will fit in this space. The full external size is 23.5cm x 25cm and the actual cutout is 21cm x 22cm. Here is a link to it: http://www.arkpetcentre.co.uk/shop/product_info.php/products_id/1942
The normal size ones have cutouts of 16.5cm x 17cm. Which would be best? Also, if anyone knows of any other brands that would be btter, please advise.

My dwarf lop fits through a "normal" sized cat flap fairly easily, but I'm not really sure how big Rex's get. I would advise getting one with a separate plastic panel which you can slide down inside the door to lock it, if you see what I mean - I have one of these and it is great, I have rigged it up on a pulley so if I need to shut the little darlings into the shed, I can just release it to cover the flap from the shed doorway and the flap is locked before they can run back out through it again :lol:

I have also drilled a small hole in the actual flap itself and have it permanently lifted up - partly to provide a little air into the shed, but mainly because the racket they were making running through it was driving the neighbours bonkers!

My 2 Satins have a large cat flat! It is clear so that they can use it as a window too! I don't think they'd have squeezed through a normal sized one that easily! :shock: :lol:

I'm not sure of the make, but I looked around and it is one of the cheapest of that size. I got it from pets at home, it is 4 way locking which is really handy!