You want a 12" plastic magnetic folder available Ebay/Amazon/whatever for 20 or 30 bucks. I have two, one is permanently duct taped/glued/doweled open so you don't need a flat surface. Not so bulky as to be obtrusive, but plenty large enough to play chess without feeling annoyed by the equipment.
I have a couple of 7" magnetic sets and they work in a pinch, but they diminish the experience for me. I have a 14" and that's just a little too much bulk for easy carry and borders on obtrusively big at say a diner. All this based of course on my probably feeble honest opinion. 
When backpacking, bulk and weight are pretty critical. Every extra ounce is an ounce you have to carry for mile after plodding mile.
But when you're pooped from carrying all your gear on your back for miles, you don't want to deal with something tiny & fiddly, either.
What's a good hypothetical sweet spot for chess on the trail?