Martin's 12-fret 000 was originally designed for gut strings. It's about a centimeter bigger all around the the lower bout (15" wide instead of about 14" for most classicals), and uses X bracing. I've made a couple that way, and they work OK. You have to use a thinner top that you would on a steel string, and lighter bracing.
That said, Waddy's right about the cross dipole mode, and that affects the sound in an interesting way. On most classical guitars the cross dipole resonance is something like 3-4 semitones above the pitch of the 'main top' resonance. This is close enough for the dipole to 'steal' energy from the top resonance, and cut into the width of the output peak in the spectrum. It's been observed that guitars with a narrow 'main top' peak tend to have a more 'cutting' timbre. On a classical, where the strings don't have nearly as much high frequency energy as steel strings do, that narrow main top peak can help the instrument to cut'. I'll note that Flamenco guitar makers often omit the outer two fans, which makes the cross dipole even lower in pitch on Flamenco guitars, and helps to give them their characteristic timbre.
Steel string guitars have the opposite problem: there's so much high-end energy in the strings that you have to think of ways to get enough bass and fullness to balance it. X-bracing tends to give the pitch of the cross dipole up; often to seven or eight semitones above the 'main top' resonance peak. This gives a much broader peak in the spectrum at the 'main top' frequency, which seems to me to help in getting a 'full' or 'solid' sound out of a steel string.
If you do make an X-braced classical, I'd say go for something at the size of a 12-fret 000. Use the lowest density top you can find, to help keep the mass down. Use a tieblock bridge, as most classicals do these days: if you use a pin bridge it's almost assured that some idiot will try to string it with medium gauge steel strings and rip the top off. Besides, using a tieblock bridge means you can leave out the bridge plate, which will save a few more grams. Use narrow (1/4" wide) bracing, and lighten up on the tone bars: you might even leave the lower one out entirely. You'll only need one finger brace on each side, too. Don't use a scalloped brace profile. That tends to shift the cross dipole pitch up and the 'main top' pitch down: good for steel strings, but not so good for classicals. I used 'tapered' bracing, tallest at the bridge, on both of my 000 classicals, and they came out pretty well. Use a 'normal' steel string size soundhole; you don't really need to shift the 'main air' resonance down below the usual F#-G .
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anyway,