Hi Bayleigh
I'm a roadie so usually try and go for a nice snug fit, but not too tight otherwise it's hard to get your hands in the back pockets, as well as showing up those areas you might think 'need more work'!
If it fits snuggly and comfortably rather than 'tight' it will wick better and you'll stay warmer as you won't have a acre of fabric billowing around you, to get caught on branches and brambles.
However, don't worry too much. I'd certainly go for some kind of cycling jersey, rather than a T shirt as the material is better, and you get pockets on lots of them ( see Team Estrogen - there's LOAD of them). But if you don't feel happy with mega sleek go for something with a little give Whatever you feel happy with...