For warmth and dryness in any tent where you can possibly fit one, a small cot that keeps some air under your ... (low hanging parts?) is the only really satisfactory solution if there's any possiblity of rain or even heavy dew. Even then, it's difficult to keep the covers from draping down into dampness on the floor and making everything dampish. (Well, more dampish than ordinary bedtime activities...)

A good ground cloth (impermeable) under the tent and an impermeable "floor" tarp that doesn't quite touch the walls of the tent may help to keep seepage/trackage etc from accumulating in the bottom of a tent that's in good shape in good weather, but there really is no such thing as a "dry" tent that people sleep in. Ventillation when the tent is empty is the best (partial) remedy.

A good thick air matress is perhaps second best, but in our sandbur prone climes one seldom lasts more than a couple of outings, and never more than a single "season." They can be patched but only if you can find where the leak is - which usually is close to impossible. Sandburs, and other "pricklish plants" here don't quite make holes, they just make "generalized porosity" in an air mattress.

If you're camping where it's consistently dry, with little variation between day and night temps (to preclude dew), or in very cold places where the ground is solidly frozen, a "ground pad" of (usually half inch) thick foam is quite helpful.

A principal problem with the smaller "crawl-in" tents is that if you can't stand up inside, at least "straight enough" to pull on a pair of slightly damp trousers, you must be in a campsite that tolerates seeing your "dressage" out in the open. In many places this is (and should be anywhere there are campers) considered quite acceptable, but I've run into unreasonable and irrational objections. "Personal modesty" might also be a consideration, although I can't really think why. (Having nothing that anyone cares to look at, I generally find I'm mostly "invisible," to an extent permitting "necessaries," but the situation might be otherwise for otherones.)

John