As long and as fine as I need, and no longer or finer than is necessary.
Fishing wets, streamers and nymphs I usually start out w/ 7' 3X and taper down as needed, usually about 10" for each size.
For dry fly fishing I start off w/ a 9' 3X and usually taper down to 4X or 5X, usually about 14 --> 15 inches per section.
There are times, when the water is low and clear, that you have to taper down to 6X or 7X and 15' long, sometimes that isn't enough.
AgMD