The Bassoguitar by Regal. While not any bigger than a normal double bass, it is enormous for a guitar. In the days before electric bass guitars existed, guitarists wanted ways to be able to play bass without needing to take years of double bass lessons and so large fretted guitars were built the size of bass violins. They could be played pizzicato or with a large leather plectrum. A few jazz orchestras used them--Same Morgan's for one. They were also quite popular in the hillbilly string bands.
If you want to hear what one sounded like, here it is being used in this Sam Morgan 1927 classic "Short Dress Gal" (if you're listening through tinny speakers, you probably won't be able to hear it):
SAM MORGAN'S JAZZ BAND - SHORT DRESS GAL - ROARING 20'S VICTROLA.MP4 - YouTube
Not to miss out on that market, Dobro came up with a resonator version of a bassoguitar:
Gibson came out with a huge bass mando that actually balanced on a stand while played:
Gibson even came out with a bass banjo:
