Hi there!
If you go to the Photos part of this site, there’s an album of rabbit condos/houses that you can check out. You can also look up “rabbit condos” on Google (including images) for lots of ideas. Personally, I would build ramps because she will eventually get old enough where she won’t want to jump as much. For example, she could develop arthritis. If you want to provide her with jumping opportunities, you could build her a little obstacle course with treats to convince her.

As for size, you want to give her room for a litter box, whatever she eats out of (a tray or food trough), and at least a couple of feet for her to lounge and move around a bit. Both levels should be tall enough for her to stand up and stretch up without touching the ceiling. If you are making multiple levels, then I would devote a story each to the litter box, food, and exercise/lounging space. I have a pen around my cage so my Dutch rabbit can get exercise outside (he is very rarely locked inside of his cage), but if you will be keeping her in the condo for long amounts of time, then definitely give her a few feet to hop around in.
Good luck!

Have you searched the forum yet? There are tons on here. I post a start to finish of mine about 3 years ago.