What is the proper hole size to drill through joists for wiring?
According to International Residential Code, the maximum diameter of a bored hole in sawn lumber is 1/3 the depth of the board.

See this answer for more details.

A 2x8 joist has a depth 7¼", so maximum hole diameter would be 2 13/32" (7.25" / 3 = 2.41667").