There is no standard size of gravel tent pads for the Full, Preferred, or Tent/Popup loops. As
@Teamubr said just above, Premium and Premium Meadow don't have a gravel tent pad and aren't really intended for tents (although you can always put a free-standing tent on the concrete or try to find a spot among the adjacent bushes and trees).
Regardless, I recommend people call the reservation line and ask the CM to add a note to your reservation and give them the size of your tent. Say, "I have a 10x14 tent on this stay". The reservation system automatically pre-assigns campsites to arriving reservations based on what should be available that morning.
But since there are no standard sizes, you might have been automatically assigned a rather small tent pad site (there are a few that are cramped/tight). When notes are added to a reservation, a CM in the FW back office checks the notes before each day's arrivals to try to spread pixie dust and satisfy as many requests as possible. So they will check your pre-assigned site before you arrive and see if your tent will fit (they have those detailed measurements for each site. If a swap is needed, they will do it and no one will ever know it happened).
I add a note on every Fort reservation usually asking for a specific loop based on my site category that I booked. Tent/Popup=1500, Preferred=100, Premium=400. And the success rate is pretty high.
So add the note to your reservation and when you check-in, verify with the CM but it usually works out.
Bama Ed