Plugin Help How to set the spawn area

Hi,

I'm currently working on an uhc mod and I 've mad a zone where player wait until the start of the game.

I've been able to set my spawn point correctly but I've got a little problem :
The spawn Area is too big (my waiting room is 20*20 including wall so many time, player spawn on a wall)
How can i reduce the spawn Area from 10*10 to 5*5?
I've only seen command that change the spawn protection radius but it's not what i want