Older French bicycles used a 14 mm x 1.25 mm thread, but these are quite rare. French-threaded pedals are commonly labeled "D" and G" (French for "droite" and"gauche" (right and left). A French pedal will start to thread into a 9/16 x 20 crank (and vice versa), but will soon bind. Do not force it, or it will damage the crank. Aluminum French cranks are easily rethreaded to 9/16" x 20 TPI.
In the early 1980s, Shimano attempted to popularize a very large thread size, called "Dyna Drive." This system was designed to place the pedal bearing inside of the crank, so that the foot could be slightly below the pivot point of the pedal bearing. This was believed to offer biomechanical advantages, but the system was not a commercial success, and has been abandoned.
One of the best flat pedals that work well with MTB'ing is Shimano Saint PD-MX80. It has concave profile to seat your foot comfortably and adjustable pins for excellent traction. Solid, sealed bearing for heavy, wet off-road usage.
I know there are better flats out there but at $65, this is one of the best you can find. Read the reviews here.