What's the nail size for attaching 2x3 furring to joists?
The nails you want to use would be the coated style of 16d. These are the ones long enough to go through the 1.5 inch thickness of the 2x3s and still get good purchase into the overhead beams. The coated nails (slight greenish or brownish in color) are designed to really bind in well into the wood so that they do not pull out easily. 

Do not use the common uncoated type nails.