I can see a typical humanoid of Medium size, 5' threat range when armed with a Medium, one handed/maybe two handed, non-Reach weapon, going up to 10' of threat range when Enlarged, if he had a weapon like a Bastard sword he could opt to use two-handedly. (Large creature plus an weapon enlargement (M becomes L) good for 5' in reach.) But if he's not using a potentially two handed-weapon to start with, the Enlarge wouldn't do anything special for him, and he'd be stuck with 5' of threat range still.
I came across a bit concerning Reach when looking into creating balanced races from which characters might come: the first 5' is standard for what you can threaten as a Medium creature, extra 5' chunks are worth about a character level each. So if you are trying to incorporate a few PCs with, shall we say, "extra endowments", be sure and make them pay cash for the special favor. You want an extra 10' of threat range? Pony up two levels.
You wouldn't happen to be working on a 6th level Improved Enlarge spell, would you? Two extra size categories would be kind of useful sometimes. Especially if you know someone with grafted-on Otyugh or Roper tentacles..