Maybe this is obvious and I'm just being dense, but what length of slide do I need for an 18" face to back measurement?
No. It is not obvious. Like ddawg16 says, there are lots of types of drawer slides. Measurements may vary depending on slide type, drawer contruction, and mounting method. The suggestion from oh'mike regarding the type that mount to the back of your cabinet box may be the simplest.
I suggest picking the drawer slide type you want, looking at the mounting instructions (usually available online if necessary), envisioning how it would match up to your drawers and cabinets, and picking the longest that fit within your restrictions.
Most likely, the critical dimension is inside of face frame to back of cabinet. But...I would take my ruler to the nearest store and checking critical dimensions, mostly overall length. If you are shopping online, best to confirm those critical dimensions somehow.