as landarc says... do what works for you... if you only want slices, then cook the untrimmed flat.
that said, you can fit a full packer on a 18.5" WSM with a little curl on the end (or as some have done drape it over something until it shrinks a bit).
You can slice a full packer too...
Lonestar 24" Vertical offset w/ Insulated Firebox & Performer Platinum
Former owner: Jambo Backyard, Klose BYC, Med. Spicewine, Pitts & Spitts, XL & (2) Med. BGE's, 22" WSM & (2) 18" WSM's, 18" & 22" Weber Kettles