If you want light weight, look no further:
Just with regards to image quality though, I'd stick the FA43 on your list.... (It's small, and brilliant)
The FA28 f2.8 is pretty small, an improvement over the older 'A' and 'M' variants, and can be had quite cheaply. The FA31 is much better admittedly, but it's bigger, and costs a lot.
The FA77 is pretty much the gold standard of portrait primes on Pentax (unless you are lucky enough to have sold your left leg for an A*/FA* 85mm f1.4). If you want a lower cost option, then a DA70 f2.4 limited is worth considering (It's about the same size as the FA43). It's not strictly a full frame lens, but it does cover the FF sensor (you may need to remove the lens hood though)