I used 5/16" grade 8 bolts, grade 8 nuts, grade 8 washers, and grade 8 lock washers plus blue locktite . Bolts for the dual sprockets were 1 1/2 " long with threads only on the lower half. For a single sprocket I bet an inch would do it but you better measure first.
In my opinion grade 8 is overkill but since they were available I used them.
Or you could go with 8mm bolts, nuts, etc. That would do it too.