My husband also designed my ring and had a local jeweler make it. We told him what quality of diamond we were looking for and how much we wanted to spend. He went out and got a bunch of diamonds that were in our price range and quality and hubby and I picked the ones we wanted for my rings
It's not always more expensive to do this. Hubby and I were also in college when he had our rings made for us
I agree with everyone else, go for quality, not size. If you can do both, then great, but if not, go for quality. If you decide to trade up, this will be a much better investment.