The same size as already delivers and pours wet cement. Powerful electric motors are compact these days, most of the size issues are batteries, which a cement truck has plenty of space to mount.
“which a cement truck has plenty of space to mount”
Space isn’t the issue. Weight is the issue. Drop a few tons of battery on a truck that is designed for a certain carrying capacity, and you are either going to have to reduce the amount it can carry, or give it more horsepower. More horsepower means a bigger motor, and an even bigger battery, and so on...