Generally speaking, you want approx 12″ of overhang at each end of your hammock for good protection with a hex shaped tarp. This is not quite as important with a Winter Tarp because you can always close the doors at the end to seal it up better. Some more experienced hammock users choose to minimize tarp coverage in order to limit the weight of their gear while others will choose a larger tarp providing lots of coverage as well as plenty of room to hang out under their tarp.
An 11ft hammock will have a 110″ long ridgeline give or take. Going with an 11ft tarp will give you approx 11″ overhang and a 12ft tarp would give you about 17″ on each end.
A 10ft hammock will have a 100″ long ridgeline give or take. A 10ft tarp will give you approx 10″ overhang and an 11ft tarp would give you about 16″ on each end.
The other factor is the height that you have to hang your tarp because of the angle of your hammock suspension. A very long tarp is great, but you have to hang it higher to get it to fit and this reduces your coverage on the sides. A smaller 11ft tarp will let you hang it low near the hammock when the weather turns real nasty.
Below is a drawing that illustrates the effect of tarp length and hanging height. (This is not our drawing. We just found it on the internet.)

Here at Dream Hammock, we personally use and recommend tarps made by UGQ Outdoor LLC. They offer a full selection of different sizes, models, fabrics, etc. Shop UGQ tarps here.