I believe they're 12x1.5mm if I recall correctly. just measure across. If they're just slightly less 1/2 inch that would be 12mm by coarse (1.5) pitch. If they're halfway between 5/16ths and 1/2 inch then they're 10x 1.25 and if they're bigger than 1/2 inch they'd be 14x1.5.
Or if you have a set of metric calipers you can use that to measure both the thread pitch and the cross section.
These will pretty much hold true with most factory installed bolt sizes on nissans although the thread pitch dimension may vary I've found alot of the aftermarket 8x1.25 bolts tend to have 13mm size hex's and the 10x1.25's often use 15mm sized hex's. Personally I like to replace as many things as I can with socket head grade 12.9 bolts because they're alot stronger, and they look cool too (to me at least)
Also if I flubbed any of these up someone please correct me.