Sureflap - how big is a small/medium size cat?!

[DontCallMeBaby]

We need to bite the bullet and get a microchip catflap - the Norty Black Cat (NBC) is becoming too much of a problem. This morning one of our cats was upstairs being annoying (trapped inside, it turned out) and when DH went downstairs and disturbed the NBC the other cat quickly appeared (trapped outside, apparently). Bad NBC.

We've ruled out the PetPorte on the grounds that we can't have battery back-up because it will be wall-mounted (local power supply is too unreliable to have only mains operated); and too many reviews saying it can be broken by brute force.

However we're a bit concerned about the size of the Sureflap - it's advertised as suitable only for small-medium cats, and is 1cm lower and 1.5cm narrower than the existing flap. Our boys aren't huge, but neither are they tiny. The bigger of the two weighs 4.5kg - they're only 2 years old, so middle-aged spread hasn't set it yet (they're neutered, so no doubt it will).

Does anyone have a Sureflap installed, that can tell me what their cats weigh, so I can compare, and get an idea of whether this is going to work for us?

The NBC, incidentally, is definitely NOT a small-medium cat, so ironically is probably going to be prevented from entering the house due to the smaller catflap as much as by the clever technology!