If I may interject, discussing potential surprises like this will kill the mood and ruin your big reveal later. So unless you’re sure your partner will take it well, I suggest not discussing things like this do just do as you please.

Some other friendly tips: you can secretly measure your partner’s finger size to get the perfect ring without them suspecting.

Going through the effort of getting the ring and sizing it properly without them suspecting shows how much you care and will yield you a better outcome than discussing this matter directly with them ;)

Edit: this is obviously with the assumption that you’ve at decided to get married. The surprise is about the ring not the marriage.