Proposing to HIM?

My SO and I have been together 3 years this June! We've talked marriage from the beginning. I tease him all the time about when he's going to propose to me. One time he said he will just give me credit card and to keep it under a grand for a ring 🤣😂 then the other day I jokingly said that I was gunna buy him a ring and propose and he was all for it!! So now I'm thinking I'll buy him a gold band and buy myself a ring that I've had my eye on for a while

It has both of our birth stones in it.

But how do I go about proposing to him? Or what size to get the ring? I'm not super traditional and I love the idea of proposing to him, just need some ideas on how to go about it! ❤️