The entrance and exit must be placed alonside the wall facing out, thats the only requirement. Maybe the square that indicates the direction is clear.

The min size is highlighted in the building size text, if its yellow its too small, if its red its too large.
Adding the size restrictions to the tooltip would make it massive so I'll look into making the build area more clear as to the limitations.

Thanks!

I'm not sure if maybe my game was just bugging but I would have to spam click for it to eventually place the wall, had to click the same place 5-10 times for it to eventually place, I had already had to quit the game and relaunch because I had people get stuck in position so I guess I could have just come across a bug.

For the size I thought it would have been good to have it tell you the size eg- 20x30 since once when trying to place a room it went from room to large, made it smaller by one black and then room too small. Maybe if it had an actual set size it would be easier.
Only my opinion of course. Have a good day and good luck with the game

Maybe its you i have no problem placing them. Just rotate them to face outside and place it somewhere.