1.85 or 2.15 rim

1,85" is the correct width for a 250F.

I don't know why 1.85 are not a option when buying after market rims such as TUSK 2.15 only for them, but 125CC and 250 bikes are not overly powerful so a help in decreased roating mass will help. I would stick to what is stock. I rebuilt my 125 with stock wheel size and different hubs and spokes.

I would put a 2.15 on a 250f. I ran nothing but 110's on a 250 thumpers, they certainly have the power to turn them.

When I used to race flat track, we always used narrower rear rim widths on short tracks. Helped get more edge of the tire on the ground, as well as making direction changes easier. A wider rim tends to make the bike drive straighter through the corners.

Wider rim on bigger tracks, to get the foot print of the tire more fully on the ground, as the corners were not as tight on bigger tracks.