How do I compare tiles to tell how many squares make a room

I am working on a dungeon keeper style game.

I don't have problems with AI or multiple instances

I have a claim tile, digging and a build system, where im happy with lairs and hatchery.

However i want to now make a training room. Ideally with code to compare surrounding tiles to deem room size. like 2x5, 3x3, 5x5

for example, if room size is 3x3, then create a training tool (if u have played dungeon keeper, im sure you'll know what i mean).

also, anyone know an easy method to change tiles when connected (like rpgmaker with grass connecting)