Rule of thumb for custom shirt diys: -If you intend to use a) white b) paint buy a men's shirt. Most paints will only work on cotton / non-artificial blends and cheap basic shirts for girls usually have some sort of stretch and the all cotton ones are either basically transparent or cost 10 bucks. I'm cheap. I can get two guys shirts for that price. For sizing I roughly guess a guy's small is a girl's large. Keep in mind that guys shirts are basically rectangles while girl's shirts flare out a little bit. Guys do generally not have super wide hips. They might look a little silly on you first. What you can do is slit the side seams open just a bit and flip em over and sew them with more space in between. Still working on that though. You can take in the sides but I love that 80s silhouette of large shirt tight highwaisted pants. I like to roll and fix the sleeves because a) cool grease vibes and b) men's sleeves are ridiculous. Ok at least their shirts do have sleeves that deserve that name but somehow they are super wide and tend to flare out like little pagoda roofs. Why? Not a good look on anyone. Actually if your sleeves do that and you're usually buying men's apparel you might be wearing a size too large. Oddly enough most men do. The collar is the worst part. I know some people do keep the knitted bit, cut it off clean and re attach it. I usually throw it away, draw a new shape from an old shirt cut and sew. It doesn't look as polished but who cares. I sometimes even hand stitch because I'm that lazy.