We have 2 twin XL adjustable beds with the legs set at 13" from the floor (easier on the joints to not have to bend so far). I tried one set of highly-rated twin XL spreads from Wayfair, but they were ridiculously short on the sides--and I REALLY don't want a dust ruffle. I'm also not into that IKEA look with all the bedding tucked in so you can see the base and the legs (I have a few boxes I like to keep under the bed).
So here are the dimensions I just measured: for the length, somewhere from 95-102" and for the width, 73-78". Those minimum measurements are from the top/head of the bed over the bottom to 4" below the base. And the width is 4" below the base from each side. Longer would be preferable, but I'm not sure I'll be able to find it.
Our old king bedspread fits fine in the length: I thought that perhaps I could buy 2 and then cut to fit--but the corners are going to be a problem as they will stick out much further than they should, and I'm not a good enough seamstress to figure out how to fix that.