I have shoulder-length curly hair and used to use a shower cap from Walmart. It was way too small, of course, and always left my curls stretched and flattened, so I decided it was time to invest in a huge shower bonnet that could actually hold my hair.
I'm so glad I did! My curls stay intact under this and my hairline has even improved because the opening isn't too tight like my previous cap (I'm pretty sure I was developing traction alopecia from the last one. Yikes). I love this bonnet and would highly recommend it!