That would be a 31.8mm
You also need to check whether the front mech is top pull or bottom pull ie the gear cable goes into the mech at the top, or is routed under the bottom bracket and goes in from underneath.
if you inadvertantly get the wrong size (as i did), the band on front mech on sora/tiagra (for definate, maybe 105 etc too) uses a common part to band to the frame. you can (with a bit of fiddling) take off the new (wrong) band and replace it with the old (fitting) one.
i didi that when i upgraded from sora to tiagra, and was lucky that the spare band fitted a steel framed bike i subsequently put the sora groupset on.
You could just measure the external diameter of the seatpost, using a flexible tapemeasure, or wrap a piece of string round it and measure with a ruler, etc.
31.8 and 34.9 are the metricated versions of 1 1/4" and 1 3/8" - usually roadbikes are the smaller size, but some carbon or alu bikes do use the bigger tubesize.
Anything bigger, or aero-profile or square-section tube, etc has to use the bolt-on variety.