here is a pic with some lenses for size comparison, all in length with NEX adapter, so your Fuji adaptor shouldn´t be much different
if you want to go light, volume also matters, maybe this helps in your search
the lenses here are left to right
Helios 2.0 50mm , old preset version - the never ones without preset are 3-4mm shorter
Rollei (Zeiss) Color Ultron 1.8 50mm
Nikkor 1.8 50mm AIS (the one with 0.45cm short distance)
Konica Hexanon 1.8 40mm - i forgot in my previous post to mention it, a very cheap way to get a compact lens
Leica Summicron C 2.0 40mm, here with a step up ring to use std filters and sunshades
if you go into the price range of the Leica, there is another candidate, the Tessar 2.8 45mm pancake, similiar size mounted as the Konica, but i just do not find it in my glass heap
the Helios does not absolutely need a shun shade, it is built with a "long tunnel". The others need sunshades what adds to the shown size
...edit: sometimes for portraits you do not have the 1m distance you normally need for RF lenses, so be aware, that f.e. in a restaurant the finest Leica style lens will cause you to leave your position, where you easily can stay with a lens going down to 0.5 - 0.7m