Is 18 inches wide enough to plant roses?
Flower Bed Width
Certified GKH Gardening Expert
That would be a tight space for roses. You may be able to get away with it if you use a climbing rose on a trellis.
Any of the knock out varities that are kept under control will work. Prune as required and although you may loose some blooms, knock out bloom contineously. Since you width is limited, be sure their soil depth is deeper than normal so that they may establish a good root system.
If you look at the growth habits of some miniature roses, the 8 inches might work. Look for short and compact growth habits. Some of them are nicely fragrant too. Again, do your homework on them for such limited room before you buy.