Hi Regeaux, welcome to the Forum,
The largest cable that can connect to the terminal block, inside for the KID PV and battery connections is #10 AWG THHN building wire (NOT Fine Strand cable).
It is possible, that the chassis that the KID can mount into might have the capability of accepting larger cable. If not, you could use #10 AWG from the KID terminal block, to your battery circuit breaker, and a larger cable from the battery side of that breaker, to the battery. Larger cable is usually better, especially on 12 V systems.
FWIW, Vic