That's corrects...If increase the conc, particles will be bigger that why the color was different. Not only reducing agent and Au solution conc and TEMP also IMP.

Yes There will be anegative effect because the size of particles will increase and also the temperature may have an effect

The mechanism is the following: once you add sodium borohydride Au ions start being reduced first to Au* (very fast) and nucleation starts, further addition of sodium borohydride solution brings about growth of NPs. This is the point: if you use low concentrations of sodium borohydride solution (like 0.01M for example), you particles will be quite large. On the contrary, higher concentrations bring about to very fast reduction and very small particles. In seed-growth method, for example, the seeds are very small and the solution is kind of black...What about amont of sodium borohydride...I think once all the gold is reduced, excess of reductive agent will be decomposed...Or the reaction will be very fast. Anyway, the NP size depends also on the rate of stirring and other factors.

Good luck!