An overhang of an inch (2.54cm!) or more will defeat most climbers, apart from very small animals. Small animals are better climbers than larger animals because they weigh less. At a certain weight, the mass of the animal breaks the surface tension of the film of water between the belly of the animal and the glass, causing it to be unable to climb. So the answer is not size, but mass - weigh 'em!
What weight? Dunno. A fully grown Cynops orientalis is capable of climbing glass in humid conditions, whereas fully grown Pachytriton / Paramesotriton / Pleurodeles / Tylototriton are not, so somewhere in between those.