Urriak Uruk
Gaming is fun, and fun is for everyone
Well, given that half-giants are a thing in D&D, and not just in Dark Sun either, they're setting-independent per the 3.5e Expanded Psionics Handbook, the idea of a human and a 17+ foot tall being mating is absurd.
With a "large" giant that might be 8 or 9 feet tall (on the low end, for a Hill Giant), that's not going into the realm of the absurd with regards to physical possibilities.
As was noted, "Large" in D&D 3.5e means anything from 8 feet tall to 16 feet tall. The tallest human alive, according to the Guinness book of Records is 8"2', and could count as "Large" per 3.x rules. The tallest man ever, at 8"11' almost certainly would be "Large" size.
An Ogre or Troll might be 8 or 9 feet tall. A Hill Giant might be 10 or 11 feet tall. . .a Fire or Frost Giant might be 14 or 15 feet tall. All size category "Large" for rules purposes.
It's much like how a four foot tall Dwarf, and a seven foot tall human (who in the real world would probably be asked if he's a basketball player) would both be size "Medium", size categories are a broad range for game mechanics purposes.
To further illustrate my point, this is literally the official size chart used in the Player's Handbook;
You can disagree with that all you like, and it won't matter anyway because you run 3E. But in 5E, giants are quite clearly Huge (they're literally the textbook example).