Common saddle clips are 7/8'' they clamp onto the top of the seatpost, if there is no difference between the top and the bottom of the seatpost, it may be 7/8 '' - 22.2 mm ..
there are decent chrome plated chromoly seatposts in 7/8 and 1.0''
from SBS/redline, that are strong, work well when extended quite a ways out of a frame..