Mike and I talked about marriage but never about rings since I never wore rings and rarely wore jewelry anyway. He knew that I liked silver better than gold, though. He went shopping with his mom and they didn't know my ring size since, well, I didn't even know my own ring size! She knew we were similar though and went 1/4 size up from hers.
He ended up getting me a gold ring but I love it! The size was perfect, it's beautiful and I love it.
When our best friends were getting engaged, Jake and I actually went shopping first. We looked at some rings, he had an idea of what Sara wanted since she talks a lot. He knew she wanted something in a "different" shape. They talked about marriage a lot, knew they wanted to get married, but Sara can be oblivious to obvious signals, so that was a plus. Jake was able to ask questions without her realizing anything was going on. (She's great with puzzles and picking up clues in books, shows, etc... but real life? Not so much. It's easy to gather information from her!
I grabbed one of her rings while we were walking once and wore it "just for fun" and kind of figured out her ring size. (I think Jake ended up taking one of her other rings with him though.) And then Mike and Sara went to a baseball game once and she just happened to talk about the type of ring she would want if Jake were to ever propose, so that was covered too. Also, because Jake has bought her other jewelry, we get the Kay Jewelers catalogs at home, and she always says what she likes!
She was 100% surprised when he ended up proposing to her and loved the ring, except for the fact that it was just a little big.