Since I don't necessarily go for maximum efficiency, I don't care much for "ideal" fortresses. At the beginning, I build as much as necessary plus a few rooms as reserve for that extra-large migration wave. Once everything works as intended and I'm at the population cap, I build as big as I want can. Barracks can be 3x3, you can cram 25 workshops into 15x15 tiles and giant walls, towers and temples for all the gods are absolutely unnecessary, but they're fun to plan and build. Having 100-200 dwarves and keeping them safe and happy is not that hard in most environments, but Dwarf Fortress is one of those games that only starts to be Fun (with a capital F) when you make it more difficult than it needs to be.
The most efficient layout is probably a vertical one with one central staircase and all the rooms as close as possible to that. If you want to make it as small as possible, 35x35 and 8 z-levels is plenty for all the farms, workshops and bedrooms to support 200 dwarves with a diverse selection of jobs. Smaller is always possible, but at some point you'll have to dump individual rooms for your dwarves and most stuff that makes them happy.
A typical basic fort contains a 2x2 room for every dwarf, 11x11 training barracks, 11x11 dining room, about 150 tiles of farms, 3 stills, 2 farmers workshops, 2 butchers, 2 tanners, 2 kitchens, 3-5 craftsdwarf's shops, 4-6 masons, 5 smelters, 3 forges and at least one of every other workshop. In addition to that, there are noble's quarters, burial chambers (just 11x11 rooms full of coffins), a luxurious jail and of course various stockpiles as needed.
Recently I build everything except the bedrooms in 11x11 rooms because such a system makes planning and designating easy. Such a room can be fitted with 9 workshops (walkable path between them) or 4 farmplots of 5x5 tiles. As a stockpile, 121 tiles are also a good size and if more is needed (mostly for food and furniture) more rooms can easily be added. The central room of each level has a 3 tile wide spiraling ramp like this:
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Stairs could be faster, but I like the trade depot to be on a lower level and have full wagon access.
Additional stairs are only added where they're practical. For example, the food-related workshops are right under the farms and both rooms have stairs at all corners for fastest possible access. Similarly, mason's workshops are on the level between stone and furniture stockpiles with stairs between them, so there's no need to carry everything to the spiral ramp and back.