My Flemish is now about 2o lbs, he is HUGE and only 5 months old. I want to take him for a walk but I don't know what size harness to get him or how to put it on. any advice?
You would probably want to go with a dog harness. The cat ones probably won't fit very well and would be too thin for such a large rabbit.
There are 2 types that would work
This one is more similar to the cat ones and I believe that some people use this style for Rabbit Hopping. I would make sure that the front strap is not too loose or get one that is adjustable (you could also buy a clip and adjust it yourself).
This type can also be used. Be careful as the front part can be quite loose on a rabbit, so make sure it is tight enough that a paw can't get stick. These ones are more common.
You could also make your own. You can use webbing from a fabric/craft store or buy some cheap dog collars and work them so it fits right. You would be able to make the H style harness in the right size.
I suggest taking your bun to the pet store to try them on. I took my girls last night and it worked very well. I put their fleese blanket in the bottom of a cart so their paws couldn't fall through and then drove them around. They loved checking out the store and people petting them.