Is she blonde.. in all seriousness that’s stupid

I'm not entirely sure I would believe that reasoning-- if it meant that much to her, that never should've crossed her mind.

Consider that bullet dodged...

Оh, you poor naive thing :-( I am sorry, dude. She should've had decency to give you the actual reason.

Take the ring back, and spend the money on yourself. Then, if you still want to marry the silly girl get her a ring from a gumball machine.

If that is what it took for her to say no, run. (She could have suggested a smaller ring if she was concerned.)

drop her and find someone who the ring does fit.

I would say you accept her rejection and you run, you run far far away. This may seem like a bad thing, but it is a great thing. You didn't marry a girl who is petty and ridiculous, and that saved you months or years of day after day of "Oh sweet Jesus, why did I marry this woman".

Take the ring back for a refund and buy one that is exactly the same size as her sister’s. Take the savings and hire hookers until you get married. Win-win!

If the size of the ring (big or little) is more important than the relationship, then she’s simply not ready for an adult relationship. Before popping the question you talk about the future and what you each want and expect out of the future. It might be more dramatic to surprise your intended with a ring and a proposal, but it’s not the wisest way to go. Talk it out first, then go to the jewelry store together to pick out the ring. Ideally you see the jeweler before hand and let them know your price range so they show you appropriate rings. Everyone is a lot more pleased in the long run that way.