Tips for framing landscape photographs...
These are my personal preferences:
Personally I like 2-3" of matte on all sides of the print which would suggest a 16"x20" frame. I would keep the same frame size for all prints in case you want to swap them around in the future.
Are you going to cut your own mattes?
I think that very few images look good in coloured mattes and my preference is normally for white or near-white colours (e.g. cream). However I sometimes use a black or grey matte. It is really a question of trying out white or black mattes against each print and seeing what works best.
I would go for a metal or black frame. Normally you don't want want the frame or the matte to distract attention away from the print.