30s, definitely 30s. A 35cm will feel enormous to you. I am 5'2" on a very good day, wear heels and still feel the 35cm is a bit large. However, if you carry a lot of stuff, you will need a 35cm. I think it more depends upon how much stuff you carry vs. how large the bag is. If you need to carry a laptop or papers without being folded, you need a 35cm. If you don't, the 30cm will serve you well.