I have a 28' Trailer and I'm seriously considering moving down to a 20'.
Just my wife and I. We don't require a large trailer anymore.
I'll eventually downsize so I can fit some of the smaller camp sites

I would like to see a couple of 'Reservation sites' at National Forrest Camp Grounds.
I would hate to travel several hours to a Campground only to find it was full when I arrived.
(I try to arrive on Tuesday mornings because I can find openings at some of the better sites.
I like to call it the joy of 'Retirement')

If you go to recreation.gov you will find many campgrounds listed that you can make reservations. Hope this helps.

We had a 23-ft TT for a few years - it was okay but it is nice not to have to step over the dog or pass each other only side-ways in our 31-ft MH.

Does Recreation.gov provide the size of parking aprons? I think so but not sure. I do know www.forestcamping.com does provide the "Average" parking apron size which might help reduce your concerns. I would say, while there are some tiny parking aprons out there (usually old CCC designed and built), the majority of campgrounds will be able to accommodate a 20-ft TT with no problem.

Yes, Recreation.Gov does list parking size plus a lot of other info, this is from their site.

Site Details:
•Site Type: STANDARD ELECTRIC
•Site Reserve Type: Site-Specific
•Site Access: Drive-In
•Checkin Time: 1:00 pm
•Checkout Time: 11:00 am
•Type of Use: Overnight
•Electricity Hookup: 50
•Min Num of People: 1
•Max Num of People: 8
•Pets Allowed: Domestic
•Min Num of Vehicles: 1
•Max Num of Vehicles: 2
•Driveway Surface: Gravel
•Driveway Entry: Back-In
•Driveway Length: 66
•Max Vehicle Length: 40
•Shade: Partial
•Site Length: 66
•Site Width: 20
•Tent Pad Length: 46
•Tent Pad Width: 30
•Capacity/Size Rating: Single
•Site Height/Overhead Clearance: 20
•Tent Pad: Y
•Campfire Allowed: Y
•Condition Rating: Prime
•Fire Pit: Y
•Grills/Fire Ring: Y
•Location Rating: Prime
•Picnic Table: Y
•Privacy: Y
•Quiet Area: Y
•Site Rating: Standard

Please remember this information, and there is a lot of valuable data there, is only for the campsite that accepts reservations. In theory that is up to 60%, and maybe more, of a campground's total number of campsites.