If a guy is particularly large, entry and sex can be a bit more tedious for everyone involved. Just make sure you take it slow. Live and learn. If you've previously been initiated into the joys of making out, feeling up, and giving and receiving oral sex, then sex itself is a whole lot less intimidating. You're already comfortable with squirts, spurts, and other bodily functions.
Sex is sex, light or no light.
But it may help to know that many first timers recommend darkness. You'll both probably be less self-conscious that way. No crowded house. Make sure your door has a lock. The last thing you need is your roomie or anyone else detracting from your first bout in bed.