Your volt drop over the wire is the current through the wire multiplied by the wire's resistance. So keep in mine that if those photo diodes don't put out much current than you won't need very large wire. A guess would be that the smallest diameter wire you can find will be much bigger than needed.
I used #18 for my led fixture and I'll be pumping 1.5+ amps through it. I can't see those diodes pushin more than a few hundred milliamperes. Got the data sheet for them?