Hi..

How big is that slug in the picture?

Well the big brown ones can grow up to 5"..

I live in the desert of Washington State.

Wow.., didn't exactly know where Washington State is and how
different it's climate exactly is but than I luckily found
this link:

http://en.wikipedia.org/wiki/Washington

Use to be all
we'd see were these tiny 1/2" whitish anemic looking slugs
under a rock now an again while I was growing up.

Well we have these nocturnal little critters too but believe
me they're innocent..

The enemy is the brown/black/red_brown fraction..!

Welcome to reality..! ;-)
--
cu
Marco