The six Elder Titans are never described as being physically big in ancient sources, like a "giant." The were primordial gods and their physical size was not important. The same is true, in fact, for the Olympian gods as well, who can change their form at will.

That said, the "Titans" are often confused with the "Gigantes," who are physically large. Though both groups are born from Gaia, they are not the same, and the Gigantes as a category are both confusing and often confused. Zeus allied with three of them, the Hecatoncheires (Kottos, Briareos, and Gyes), who are described by Hesiod as monstrous in size (μεγάλοι τε καὶ ὄβριμοι, "big and mighty," Theogony 148). These are often called "Storm Giants" in later literature. There are others also, some of whom are described as large (like Typhon).

I'm not really sure how or when the term "Titan" became associated with large size independent from "god," but the Elder Titans are not really big scary monsters. They are in fact very very similar to the Olympian gods. Some of the gigantes are big monsters, like I said, and they come from the same parents (variously Gaia and Ouranos, or the blood of Ouranos when his bits are sliced off), so there is a lot of confusion.