Line lengths are usually measured in words or characters per line (or at least in my experience). I don't think print and digital are any different — the same general rules of typography apply. Short lines can create design issues and slow reading down, long lines can be unwieldy.
This study may be helpful:
This study examined the effects of line length on reading performance. Reading rates were found to be fastest at 95 cpl. Readers reported either liking or disliking the extreme line lengths (35 cpl, 95 cpl). Those that liked the 35 cpl indicated that the short line length facilitated "faster" reading and was easier because it required less eye movement. Those that liked the 95 cpl stated that they liked having more information on a page at one time. Although some participants reported that they felt like they were reading faster at 35 cpl, this condition actually resulted in the slowest reading speed.
The Effects of Line Length on Reading Online News
For slabs of body copy, I like about 70 characters per line, but anything in the 50-80 range seems good. I also think justified text is harder to read, due to the lack of unique shapes to track on the right side of text — it's far easier to lose your place.
It's also worth keeping line length in mind when designing fluid layouts. Maximum column widths are a good thing if you want to maintain legibility.