What size seetrock for basement

Standard is 1/2 inch,unless you need to enclose a furnace/hot water heater area,then it is 5/8 inch,for code.

Also check your local codes for minimum thickness and fire ratings in you local area,they might be different than this.

Are you asking about thicknes or length/width? Also, I'd consider greenboard or paperless drywall so you will have less of a worry regarding mold.