What ya using?

Is this not entirely dependant on frame size and inside leg measurement?

There’s a chart on here that might be of use?

http://www.bikeradar.com/forums/viewtopic.php?t=12971373&p=18900218

150 Reverb Stealth on my old one.

Tape measure, current post set to pedaling height and common sense, surely?

Once you know exposed seatpost length at normal ride height, Alltricks have a useful chart for figuring out what you can use…