I had 15" and it was too narrow by about 12" to be really easy. I would think 18" would also be about the same. If I had a good wide table next to my cooker, this might not have been as big of an issue. I really think 24" is the minimum, that being said, I am cheap enough, that I would buy the 18" over paying shipping on anything wider.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."