their size guide seemed to involve having their kit modelled by staffers with a range of unusual sizes….
For their base layers you seem to need a different size depending on their weight, IIRC I have a large 150grm and a medium 200/250grm and all are baselayer tight (might be other way round, not in a position to check right now).
Why do you use merino tops for something else? They are pants for riding.
The use/value of using merinos as base layers seems to be a function of how heavily you sweat. All the naysayers in recent threads complain about them soaking up and holding sweat so you end up getting cold; I wear them as a base layer all the time riding mtb, road, motorcycle and snowboard and think they are brilliant but I’m not a profuse sweater (I’ll have a red glow when riding buddies have it literally dripping off them).