If there is a rescue near you, you could try some bunny dates. If she responds well, they she might want a friend. If she doesn't respond well after a few different rabbits, then she may not want one. Bunny dates are also good to try to find a friend for her and to let her choose who she wants.
Size doesn't really matter, but a very small and a very large rabbit might have some issues. It can be done though, you just need to consider the needs of both rabbits. I would try to go for a rabbit of a similar size, but don't rule any out just based on size.
I would keep the separate for a bit so the new rabbit can adjust and you can monitor for illness. If the new one needs to be fixed or has recently been done, it would be best to keep them apart until he is healed. When they are ready to start bonding, start on neutral territory. You should be in the area to break up and scuffles or fights. The are should be big enough that they have room to get away from each other, but not too much space as you want them to interact a bit. If you are unsure of how they will get along, have 2 pens so they can sniff through the bars before you let them face to face. As they get used to each other, the bonding sessions can be longer and you can eventually move to the area where they will be living. When you get to the area where they will be living, you should back up a few steps as your rabbit might not like having someone else in her territory.
Generally, males and females get along best. Female pairs can also be done. 2 males can be done, but it is tricky. Both should be fixed and given time for the hormones to calm down before you try too much. As you have a spayed female, a neutered male would probably be best, but a spayed female could also work.
