My 12 inch foldable wooden sets are all absolutely fine to have a friendly game OTB.
I also habe one of those cheap 10 inch black and silver foldable plastic sets, that I used to play correspondence chess with a friend, and it was perfect for that. I could probably play OTB with it, but I would not go any smaller than that.
I also have a 5 inch foldable wooden set that I bought just for fun. While it is magnatic, the magnets are very weak and the pieces are so small that it is hard to pick one up
without moving another piece. It's cute, but useless. I don't think 2 inches more would make alot of difference.
10 inch ok-ish set. Also very cheap and probably available in 7 inch as well.
Too small 5 inch set:
My most recent purchase was a Venture Board. I mainly use it for opening and endgame study. However, i would like to use this to play others when I travel. I cannot read the position when my opponents pieces are turned around. Any suggestions on smaller boards you have used to play with smaller 3D travel chess sets? 7 inch square max please. Please ignore this post if you do not have experiences with these types of boards. Thank you.