Here is a classic image that shows a typical Corps or Army Field HQ tent. It is MacClellan's at Antietam when Lincoln visited.
Note how large it is. He's using a typical hospital tent, rather than a more common wall tent, though both have the same proportions and layout. Especially note that the fly is stretched and staked OVER to tent, rather than as a "front porch" like you see so many reenactors doing.
The purpose of the fly was to act as both insulation and extra water reppelancy, and not to extend out in front of the tent. There perhaps a dozen period images that show tent flys put up in front of, or to one side, of a tent, and in every case, you can see that these are additional flys, with the one accompanying the tent being set up over the top as it was designed to do.
As both wall tents and common wedge tents came in a couple sizes, you can use any designed for 25/28mm for 20mm with perfect ease.
One thing to note is that with any command tent, the higher up you go, you usually get an extra tent for the HQ. The 1863 regulations specified that at the regimental level, only 1 wall tent was to be used for the regimental HQ. 1 was also given over to the surgeon for hospital use. Going up the ladder, it would be expected, as I said, to see one additional wall tent for each level, so brigade=2, division=3, etc.
Having said all of that, it is VERY doubtful that even the mighty Stuart would have had more than 1 wall tent, as they were quite scarce in the Confederacy. Most tentage, where available, consisted of the tent flys themselves, or just large squares of canvas like painter's tarps with some grommets sewn into the corners and along the sides so as to make lean-to shelters, etc. Supporting these would be wedge tents, but even they were almost unheard of on campaign, as there wasn't sufficient wagons to haul them all.
V/R
ponders nirvana.
