For a run of #2 electric wire, in conduit, what size neutral and what size ground wires should I use? It's about a 50 foot run.

I come up with, #8 for Ground and #4 for neutral.

Now you got me wondering...
I agree with the #8 for ground...
Why would the Neutral be a different size from the Hot leg.
Is not current equal in all parts of a series circuit?
Just wondering...
I generally would use the same size wire for my hot leg as my neutral leg.

Reduced neutrals are allowed for electric ranges and dryers, and can be no smaller than 70% of the circuit rating.

Neutrals for feeders are not related to the wire size of the ungrounded conductors. Reduced neutrals for feeders are allowed ONLY if the maximum unbalanced current is calculated.

For a 100 amp service with #2 wire, not worth calculating or taking any chance the calcs are done correctly, and use the same size #2 for the neutral.