I would say 5x5, as I ran some fortresses earlier, and it was fine with up to 200 dwarfs with no screw pumps.
As of the river and peak - a peak I am looking for is about 15-20 squares circle at the bottom, reducing by 1-2 squares to the top, going up about 7-10 levels or more (a cone of sort). I managed to get some of these, but not the whole bunch together.
A way cooler option (my guess - also rare) a mountain/hill (about 50-70 squares at the bottom - less ok, bigger probably will be an issue design-wise) over the river, with river running some distance underground. I have seen some places like that, mostly when two rivers meet, but sadly with no savagery of needed level. Then a fortress can sit atop the mount, with walls circling it, a cool entrance directly into the side and the whole nine yards.
Nevertheless, if the river is not solely by the side or corner of the embark, it would be good too.
The idea is a tower overlooking a valley, with a bunch of bridges over the river (this is why brook is not that fun, anyone can cross it, so why pay taxes?). If a boook can be made into a river by channeling that would work too (have not researched the matter yet, just got the idea)
Edit:
If an embark is on a mountain, one can build Minas Tirith

Edit:
Fewer levels of peak seems also to work out.