How large is it when a hard drive is just too big?

That is a pretty subjective question. Hard drives come in a variety of sizes because users have a variety of uses. People who have a large amount of multimedia files on their machine will naturally need a larger drive. The answer would pretty much be dependent on how much space do you need? Are you going to have alot of programs installed at one time? Are you going to be processing multimedia files (audio/video)? Do you plan on going on a downloading spree?

If you're looking to upgrade, I'd suggest looking at what you have compared to what you want, then look for a drive that has a decent cost compared to size. Many times you can find larger drives on sale or with rebates that make it worth the purchase even if you won't be using the space anytime soon... Better be safe than sorry... don't short yourself on space, but don't spend mega cash on a brand new 250GB drive if you're not going to need the space...

depending on the use you have for the computer.

if you are only going to use it for word processing and spreadsheets, a 40 gb would probably be big enough.

if you plan to play video games, store music, probably a 80 gb.

if you want full multimedia, store or do video editing 300 gb.

Depends on what OS you are running. But you can go up to 500G if needed.