For the actual in-game size, I would say to look at maps like knightfight, canyon, maxsecurity and others to see what makes them good or bad. They are all pretty big maps but each have respective pro's and cons. Maxsecurity is a bit too open for my liking while canyon and knightfight both have some nice cover strewed around to aid the rebellion. Sometimes too much of this makes the map too t sided so watch out.
Lego is a strange one as it's pretty big but has narrow corridors which makes everything cramped. You don't want that in your map so try to provide some nice distance so ct's aren't always in knifing distance of the t's. The stairs portion of knightfight also has a pretty cramped area that ct's don't know what to do with it and a lot of them die there so watch out for that.
If you're going to have a large map, walls are a great way to break up areas. Take a look at this image of my next map.
See how there are lots of walls breaking up a huge thing into smaller sections. That is what makes a map better as t's can run away and cant get mowed down from every angle but as long as you have some competent ct's, you can hold the t's well. Getting the perfect balance can be a bit tough so experiment a bit to see what works. Your map can be as big as it can be as long as it doesn't take too long to get from one end to the other end and it isn't too open.
There are also some other limits hammer has.
For the entity limit of 2048 detail and static props don't add to it.
For overlays, there is a hard limit of 512 but generally you won't be using too too much of those in a jail map.
oh ok thanks RDK. btw if u wanna know how big i want to make it, im planning of putting a forest, an aztec area, and the main jail. All of them are going to have their own seperate areas.
Just make sure it doesn't get too out of hand. Put some games in those areas so that they are beneficial for the map.
Happy mapping!