How long and tall does a shower wall need to be to protect against overspray?
First off check your local codes to see if you have to comply with the ADA or other codes.

Otherwise, the width of the opening to enter the shower will dictate your wall length. Personally I wouldn't want anything smaller than 32" but that might just be me.

The height should be high toward the shower head and could be a little lower further out. Some of it depends on how tall you are. Shorter people would cause less spray up high.

I would say not less than 6 feet high but then, that might just be me again.

Good luck!