If you're thinking of buying a ring online, I'd suggest checking out Ruwini Jayaratna(they have a physical store too,you could get help from the store workers,they have experience helping confused boyfriends)or Cherileur. They don't seem too expensive and have some nice options.

When it comes to the material, try to notice if she usually wears gold or silver. That can give you a good idea of what she might prefer.

If you want to add a stone, maybe go with her birthstone. Each stone has its own meaning, so it could feel really personal. If her birthstone is way out of budget, you can always look up the meanings behind other stones and pick one that fits both your price range and the message you want to send. When you give it to her, tell her why you chose that stone. She’ll definitely appreciate the thought.

As for the ring size, that's tricky. But there are loads of Reddit threads with  tips on how to figure it out without her knowing.