I just did some testing with a progressbar with a width of 5000. Yes, only a fifth of it was on the screen, but I checked the pixels at several points.
If it's got enough pixels, HP/Mana bars are capable of displaying the info in increments of 1 tenth of 1%, possibly smaller. I'd need someone with 5000 HP/MP to be certain.
(Progressbars won't stretch past about 6000 pixels.)
So if you like stacking your own hp/mana bars right on top of your target's hp/mana bars, you can squeeze out that last drop of comparison by stretching the bar to the full width of your screen.
Just thought I'd let everyone know.
didiputenoughinnuendointhere?