Before starting formal academic training, I taught myself basic botany by toting the 4 volumes of Abrams & Ferris’s (1923-1960) Flora of the Pacific States around the desert in a day pack, and making liberal use of the line illustrations (every species!), which remain some of the finest available for U.S. western plants. So, maybe not the most portable, but it happened, so qualifies as a “field guide” to me.
Even less portable, but in my biased opinion the finest flora of modern times, is the 9-part Intermountain Flora series, which covers the Great Basin plus all of Utah, with exquisite full-size line drawings of every taxon, and extensive descriptions and discussion. I have taken it with me to the field for sure (and it shows on the bindings), but haven’t put them all on my back, so quasi-field guide at most.