The 3.5 rules set does not classify ranged weapons as light, one-handed or two-handed, yet this classification is required to accurately determine modifiers for two-weapon fighting. Ranged is the only classification, with the exception of this from TWF:
Treat a dart or shuriken as a light weapon when used in this manner, and treat a bolas, javelin, net, or sling as a one-handed weapon
With melee weapons that can be thrown, like a dagger or spear, you get the classification from it's melee counterpart. Sometimes, you can read the description for more information, like with a composite longbow to determine you need two hands to wield it, but it only grants bonus Strength damage as a one-handed weapon. This lack of size classification for ranged weapons was carried over to Pathfinder.
There is this bit of information from Ultimate Combat
Concealing Firearms: Like light weapons and hand crossbows, one-handed firearms are easy to conceal on your person. Some smaller firearms (like the coat pistol) can grant bonuses to conceal a weapon on your person.
While not stating they are light, this does state one-handed firearms are "like light weapons", at least with respect to concealing one.
There is an example monster listing, the pale stranger that uses two pistols.
treats pistols as light weapons for the purposes of determining penalties from two-weapon fighting
However, it is among a list of Supernatural abilities that are are unique to the pale stranger, like the ability to "supernaturally reload". Whether or not their mention of "as light weapons" was intended to be a supernatural benefit, or simply adding an otherwise missing but necessary piece of information is up to the reader.