drop trap loft

Hi guys, I am new to the pigeon world. I want to build my first loft. The loft I am planning on building is 4x6 and 7 foot tall (I have read posts...im over 6 foot) with a 7x12 and 7 foot tall aviary. I really dont see alot of straight up info on numbers and dimensions. my questions...

So after looking at all the post on types of drop traps, I've decided to go with the drop style and a hinged door to close it off when not in use. My question is what is the absolute minimum spacing so Rollers can fit thru but discourage cats? Any help will be appreciated. I'll let you know how...