I have a jonart and love it. But yes and no about 48x48 inches. For whelping and up to about three weeks, the size is good for a normal GSD litter, after that, they will need more room. Mine usually have more room in the house by x-pens and baby gates, and more room out of the house by a doggy door to a kennel, and a play yard.
I will show some pictures, Jenna's litter of ten.
Outside kennel available from the inside whelping area:
Play yard for when we are out front with them:
They need their space, and Momma will need a way out:
The box -- getting a bit crowded:
The box with plenty of room:
Hard to see, but I just removed the door panels and let them have that part of the room and the doggy door to the outside as they got bigger:
And then when I was kind of training them to use the outside for pottying, I pushed the box up to the doggy door which gave them less space inside and encouraged them to do their business outside (Jenna's second litter):