Currently, 5x5 rooms (except where needing them larger, when they're often 11x11, i.e. four 5x5s with the walls knocked through), though that's a been different, and will doubtless change again. But if I really want a 5x5 and I could only get a 24-square room, without breaking through to the outside, I'll break through into the outside and rebuild walls (of blocks of the same material, not even ones just of the same colour!) and floors and ceilings above to make them so. In fact, in a recent fort I even extended the room and corridor pattern out over the hillside, like some weird Los Angeles hill-side building (sans stilts, because I didn't need them).
On the subject of same rock colour, probably my most invariant trait is that if I'm digging out something that I'll not go back into (like a cistern, or magma channel) I'll completely smooth it off before 'commissioning' it. And if that cuts through gemstones or ores (and even just touching upon a regular rock type where it's not a major component), I'll mine them out. Could be just one tile into the wall, if I can later on get the more distant items by digging in alongside, but where there's doubt I'll pursue the cluster/vein as far as I can. Then (having arranged for blocks of the appropriately local native material to be made), I'll wall the side up. Though these days I tend to leave voids behind the walls, rather than fill them in. And even if it's only one tile in, it does leave little 'warts' of visible rock beyond the built walls, where there are none behind the smoothed ones.