I’m always large on MTB’s, have been for decades and even now I’ve got a (reasonably) LLS HT, still large – felt a tiny bit stretched at first, but it isn’t – it’s spot on now I’m used to it. I’m 5’10.5″ with long legs/short torso
I was convinced I’d need a large gravel bike based on this, but wherever I looked, was guided towards a medium Boardman Adv 9.0
I went with a medium and I’m glad I did – a large would have been too stretched out (a road bike may be a different scenario). I flipped the stem (upwards) before my first ride and whilst it obviously felt very different to my MTB’s, it was fine for a 35 miler and I used all the bar positions