Please help!

There are so many cat lovers here and at least some of you must have
cat doors. I need to install one before next winter, so the cat I feed
wouldn't have to wait outside on a snowy windy porch before I spot her.
However, I don't want to wait until winter and I will have some help in
June as well.

The Petsafe Ultimate Small cat door, their best quality, has an opening
of 4.25 x 7". The small size is attractive to keep out weather and
undesired animals like racoons, but my worry is, is 4.25" enough for
most cats to squeeze in? (We may get other cats in future and wouldn't
want to change the cat door.)

If you know cats and cat doors, please respond!