I have seen plenty with three rings, to allow for mic input for headsets (not necessarily stereo mic input). There are VERY small USB hubs and plenty of USB adaptors for stereo line input. Creative Labs used to make a GREAT, high quality, external USB IN/OUT amplifier. I used it for editing professional grade audio books.Something like a Sound Blaster G3 would be VERY compact (though not as full-featured as my old rig , that had digital knobs on it).
https://us.creative.com/p/sound-blaster/sound-blaster-g3
Thx. Size is a modest factor, a larger factor is just keeping track of all the small accessories when mobile — or even at home sometimes! Sometimes MORE at home, I think!!!
;-)