The gold standard for where and what around here has long been @mooch’s blog. Someone may throw you a link. I don’t have one. I just buy what looks good on liion wholesale. I’m lazy. The two standards seem to be 18650 and 20/21700.
In your position I would look at reviews of the mod on YouTube.
I’’m a big fan of removable batteries and battery chargers. Single battery mods want 3 batteries, (because there can be charging errors and one might not be recharged by the time you empty the other) double battery mods want 4. This is so you can charge while
vaping. When a battery runs down you just swap it so infinite power for ever.
There is a wide xx650 too that looks like a “C” cell and holds about the same as a 21700. There are also “internal” non-removable batteries (which is to say ewaste) that also exist. Recently I saw an 18500 and an 18350. THEYRE really small. And pretty unusual. UL actually requires internal as they don’t trust battery sleeves (not that I blame them)