I would say to buy what you think you need, as opposed to sizing up or down
See, I’d say size up. Bought some winter bib tights off them in Med. Bit snug on the legs and lower half but fitted okay. However, the straps/braces whatever they’re called seemed very short and top half was tight getting them over the shoulders.
I went with Large and still fit nice on the lower half and much more comfortable on the top half.
Bloke at work bought some and he had to go XL to get the top half length. He’s about 6′ and thin as a rake. Bottom half feels a touch baggy but he’s happy enough.
Overall good decent and cheap kit. But ime sizing might be a bit out