1) Do most parrying daggers have blades from 6" inches - 12" inches in length?

2) Was the parrying dagger used in conjunction with the smallsword at all, in the 17th and 18th centuries?

1) I've seen a fairly wide range of blade lengths for off-hand weapons, between about 6" to 18". Which was most common in period is hard to say, but most of the ones I've seen have had between 8" to 10" blades, roughly.

2) Angelo's treatise on the smallsword makes mention that the Italians liked to use the dagger with the sword. He then shows some of the guards and how to counter it. In all of the illustrations, it appears to be a smallsword with the dagger (unlike the illustrations of the Spanish fencers, who seem to be holding rapiers). That said, it didn't seem to be a common combination.