Go true to size. (ie: I am a true size 9.5 so I buy my 991s in a size 9.5)
The 991 does NOT come in wider or narrower widths.
Comfort Factor Ratings:
Cushioning (Softness): 6/10
Stability (Firmness): 7/10
Cushioning to Stability Ratio: 6:7
Midsole Shape (Foot support): 7/10
The 991's comfort is comparable to the 992 and 993, in the sense that it is comfortable in its own special way: it has a great amount of stability and support with a good midsole shape and a decent amount of cushioning, but it isn't the most soft or plush shoe.
The 991 utilizes New Balance's proprietary Abzorb technology, which absorbs impact through a combo of cushioning and compression resistance. Combined with a supportive midsole shape, the shoe has a great combo of stability, support, and cushioning.