If water does not flow fast enough to fill your gravity feed to the washing machine will increasing the out pipe to three quarters fix the problem?

Increasing the size of the outflow pipe will only help in cases where the outflow pipe is restricting flow. In this case, there does not appear to be any restriction resulting from the size of the outflow pipe. So your answer is: No. Suggestions are: 1) increase the inflow pressure by using a steeper grade angle 2) try using pressurized water storage tank in lieu of gravity feed 3) try increasing the size of the inflow pipe if there are any restrictions to flow on the incoming pipe