First of all, if any of you make posters out of your images, what's the best render settings for that? (for turning final images into printed posters). Secondly... if you wanted to give the posters away, not selling but just giving them away like for prizes and whatnot, would that be classified as commercial use? Am curious if I could do that or not cuz I use a lot of free stuff and it's generally tagged as being for non-commercial use.

Check with your printer. If you're just doing laser output, you can get away wtih as low as 110dpi and still have a good looking image. But you set your height and width for your final reproduction size. If you're doing offset printing, set your render size 266 to 300 dpi. As long as you're giving things away and there's no money made, it's not considered commercial use.