I think of a wyvern as a lesser kind of fantasy reptile. Smaller, less powerful, and without the fiery-breath element. Perhaps an opportunistic scavenger, as opposed to the big old regular dragons at the top of the food chain. Maybe they'd pick over the scraps of dragons' kills and piggyback off the bigger guys' caloric expenditure, rather than waste their own energy in hunting and killing prey themselves. A 'lesser wyrm' kind of reputation, as a hyena might be to a lion, or a vulture to an eagle.
If they relied mainly on bigger dragons to get their food for them, then they'd probably live wherever dragons live – which could be anywhere you like. But realistically, they'd be the most successful around arid brush and savannah.
I agree that a hot habitat would make the most sense for a reptile. But bear in mind that higher altitudes will be significantly colder, so a large reptile that relies on its bodyheat to get airborne and stay there may nest around warmer foothills, crags and outcrops. (Rather than high dramatic peaks). Unless of course it's a fire-breather itself, in which case it could warm itself at will and live wherever you'd like.
To answer your question, maybe ask yourself what it's likely to eat. A wyvern could evolve for many different environments, which would dictate the size that it needs to be. Say a tiny one that preys on frogs in swampland, or pack-hunting wyverns that dive on huge shoals of fish off the coast. The environment would also dictate their shape and colour, to camouflage them from prey (or predators). Hope this helps!