There are some good threads on here ref stripping paint off frames and whats best to use.
As for seatpost size, they usually go up .2 at a time so quite a few sizes inbetween. You could take it to your LBS and they will tell you straight away or trial and error im afraid.
As for seatpost size, they usually go up .2 at a time so quite a few sizes inbetween. You could take it to your LBS and they will tell you straight away or trial and error im afraid.