Holmes is now a little under half of the moon in apparent size. It's been dimming for a while now, so you're best off seeing it sooner than later.
Celestia also works as a planetarium, although it's not dedicated to that function, so you have to do a few things to see your sky (move to your spot on earth, press Y for geocentric "orbit", press * to look 180deg back from your spot on earth, and set the time), you do get a real 3D universe you can travel to.
http://www.shatters.net/celestia/There's tons of addons for it too, from spacecraft past and future to deep sky objects, some of them pretty good looking given the graphics engine:
http://www.celestiamotherlode.net/Holmes' gotten pretty big (1.4million miles), but in the past Hale-Bopp's gotten to 2 or 3 Mmiles, and Hyakute's tail was something like half a billion miles long.
Jupiter's magnetosphere is big enough that it'd be about 8 times (iirc) the apparent size of the moon in our sky if we could see it.