The Planet X sizing guide is pretty good and if you go there they’ll do a basic “sling a leg over it on a turbo” sizing for not (ask nicely).
By mail order, ring first and ask about swapping stems; they’ve been very nice about that with me.
That said when I got my first road bike I got a medium Planet X, which was great until my back settled into a road position over a few months.
I sold it on and got a large – one of the current Ultegra ones (brake callipers and chainset are not Ultegra – R565 instead which seems to be just below 105. The crank arms are not hollow and may get changed, but the brakes are fine).
Note that th OnOne / PX sizing seems pretty consistent – if you’re a large on one bike, you’re a large on all (my experience over 4 or 5 different frames from them).