I personally, for the reasons I mentioned in that other thread, find furled sails a great deal easier to model than set ones. (In fact I've never been able to produce a set sail that I've found halfway satisfactory.) But I guess everybody's different; to each his/her own.
I continue to recommend either furled sails or "bare poles" for a model. (That article in the MSW database, incidentally, is largely reproduced - with my complete approval - from what I wrote on the Forum thread I mentioned above.) If you do want to show the sails set, though, working out their dimensions isn't actually so difficult. Lay them out on paper first. If you can find a sailplan of the actual ship - great. (Reconstructed sailplans of the Thermopylae can be found in three books by David MacGregor: China Tea Clippers, Fast Sailing Ships, and British and American Clippers: A Comparison. I may have garbled the titles slightly.) Don't assume the kit matches the plans exactly, though.
If you have to work out the dimensions of the square-rigged sails from the kit parts, it isn't hard. Fit all the components of, say, the foremast together and lay them out on a table. In the big Revell kits, the yards are fastened to the masts by convenient (if not-quite-to-scale) "snap rings." If I remember correctly, there are raised rings around the masts to locate the upper topsail, topgallant, and royal yards in either the raised or lowered positions; if you're opting for set sails, put the yards in the raised positions. You can then determine the height of each sail by measuring the vertical distance between a yard and the one below it. Subtract about 1/8" (i.e., a scale foot) or a little more, and you've got the depth of the sail.
The head of the sail is stretched between the yardarm cleats. (The yardarm is the extreme end of the yard; each yard has two yardarms, one at each end.) I don't remember exactly how Revell represents the yardarm cleats, but there should be some obvious fitting two or three scale feet in from the extreme end of the yard. The distance between those two points is the width of the head of the sail.
The lower corners (clews) of the sail are held down by heavy ropes (or, in some cases, chains) called sheets, which pass through sheaves in the yard below, just inboard of the yardarm cleats. As I recall, Revell repesented those sheaves as holes in the yards. Measure between the two holes. That's the width of the bottom of the sail.
The course (the lowest sail on each mast except the mizzen; the lowest sail on the mizzenmast is called the crossjack, or crojack) doesn't have a yard at its foot. Its sides (leeches) are almost vertical; the foot is about four feet wider than the head. (I'm basing that figure on the sailplan of the Cutty Sark, which I happen to have in front of me.) The clews should be two or three feet above the tops of the bulwarks.
Each of the square sails is gored slightly - that is, the foot is shaped in a slight concave curve. (That allows space for various rigging lines that have to pass under the middle of the sail.) The amount of curvature is smaller in the upper sails than in the lower. For the courses it's about three feet; for the royals it's about a foot.
The overall shape of a square-rigged sail therefore could be described as a trapezoid with a curved bottom.
The spanker is, if I remember my high school plane geometry right, best described as an irregular quadrilateral; none of its four sides is parallel to any of the others. You should be able to take its dimensions from the mizzenmast, the gaff (the spar at its head), and the boom (the spar at its bottom). The other sails you'll probably want to represent are the jibs and staysails. They're shaped like triangles with slightly curved edges. You can take their dimensions from the sailplan; they don't have to fit precisely between kit parts.
All this sounds at least twice as complicated in written form than it is in reality. If you take a look at a sailplan for the ship, or even a similar one, it should all be pretty obvious.
Hope that helps a little. Good luck.