What SFML needs are the positions of the vertices, so you need to do some trigonometry to figure out where you want the vertices to be given those conditions. The exact code you want probably only requires a couple of sin()/cos() calls.
The maximum available anti-aliasing level is determined by the hardware. I believe typical values are 2, 4 and 6, but I don't know how high it goes these days.
Edit: After some quick googling I found mentions of 16 and 32, so I guess the maximum is pretty high. I doubt most games would benefit from 32xAA though.