llil:

The gas charged cylinders are measured with regard to two parameters. (1) The length of the extended cylinder and (2) The psi rating or the amount of force measured in psi ( 10psi, 20psi, 40psi, 90psi etc) needed to compress the cylinder. The part number on the cylinder should, in some fashion, indicate the psi rating so you can compare the new cylinders with the old ones. Regardless of what size cylinders you have they are affected by the cold in an adverse fashion. Meaning the colder it is the less able they are to function as they are designed. Despite the colder weather they should be able to hold up the tailgate. I do not know if you can tell if they are leaking or not.

Hope this helps and good luck.