Sleeves can be let out, so long as there's enough overage left at the sleeve inseam. As for the wrinkles, width is subordinate to shape. In other words, narrow sleeves and wide sleeves should both hang cleanly by matching the shape and curve of your arms. This is obviously somewhat easier to accomplish in RTW with wider sleeves, which leave more room for error, but wider sleeves still don't guarantee that the line of the sleeve cleanly follows the shape of your arms. Takes an awfully good alterations tailor to get sleeves right if your arms don't match up to the RTW pattern.
Narrow or full is really a stylistic choice. I personally prefer fuller sleeves worked into a small-ish armhole, since larger armholes are less preferable functionally and aesthetically, but I like the look and feel of fuller sleeves.
Edit: Sleeves that are too tight at the cuff or narrow sleeves worn with certain shirts which bunch in the sleeve can also cause wrinkling. Anything that disrupts the fall of the sleeve can cause it. Heavier cloth always shows sleeve wrinkles less, too, since--if unimpeded--it's pulled more taut by its own weight. I'll leave it to one of the experienced tailors to diagnose what's going on here.
Edited by ctp120 - 7/16/13 at 6:53am