2x1" is quite typical up to around 8x10ft above that typically 1.5x2.5"[38x63mm]Looking at that link @Djangobanjo shared I was a little unimpressed they appear to describe the main timbers as 2x1" or have I mis-understood? Because my chicken run is made of that!
indeed tile baton size [19x38]is sometimes used on smaller structures
when you make it yourself you choose 3 or 4x2" for larger structures
on a smaller shed i would never go below 38x63