My rule of thumb is whatever the jar size is, pack 1/4 of that amount of tobacco into it. For instance, an 8 ounce jar will hold 2 ounces of tobacco, a 16 ounce jar holds 4 ounces, and a 32 ounce jar will take 8 ounces. You can pack them tighter if you want, but in my experience Virginias in particular seem to age and smooth out the best with a little breathing room.
With aromatics though you don't really want them to age, so packing tighter is certainly reasonable, but if you go tighter than 2 ounces of tobacco per 8 ounces of jar space with the wetter aromatics you run the risk of opening the jar a year or two down the line to find that you have a sticky congealed plug on your hands where all the moisture and sugar has turned your tightly packed ribbon cut aromatic blend into a solid mass not dissimilar to a fruit cake.
Another factor to consider is the relative humidity of the air where you live. If it's a hot and sticky summer with 90% humidity then whatever air you seal up inside the jar with your tobacco is going to be very humid and may lead to mold growth. On the flip side, if it's an extremely dry winter with 20% or so humidity then the dry air sealed up in the jar can make your tobacco dry out and lose flavor over time. Around 50% to 60% humidity is fairly ideal for jarring tobacco at 2 ounces of tobacco to 8 ounces of jar space, but if the humidity where you live is very high or very low when you're jarring your tobacco then you might want to pack it tighter to push as much air out of the jar as you can before sealing it up; just to lessen the possibility of mold forming if the humidity is high or your tobacco drying out if the humidity is low.