22" deep by 33" long and 18" tall under it.
Agrees, perfect place!...Plenty big for a brooder space, you could easily raise 8 chicks to 4 weeks in that. It is actually a good space since there is visual "access" to the big girls.
...but keep in mind depending on size and type of pad, the cord can take up some space.
My 24" wide pad goes to about 27" with the big clunky power connection.
Also watch your rack size, depending on how you configure it.
With just a few chicks, a smaller pad and rack might be fine.
Plan carefully.