I use a scud (freshwater shrimp) fly in size 10 and 12. I don't know if this is the same thing that you are referring to as a "grass shrimp." I think some folks go even smaller in size but I hate digging a real tiny hook out of a fish belly when they take it too deep, especially if I'm not planning on keeping fish. These two sizes have worked fine for me on bream. I could be wrong but I believe that the natural freshwater shrimp are mostly small sizes, not large sizes so anything larger than a size 10 (some places may not have them larger than a size 12) might not be effective. It might be best to go to a local bait-shop to find the correct sizes and colors of natural freshwater shrimp for your area as I am sure bait-shops should have the live freshwater shrimp on hand. I have not caught any Specks on scuds yet though. I either use the scud on a straight fly line/leader or I use it as a dropper about a foot or two tied to and behind the hook bend of a surface fly. A straight hook may work just as good or even better than a curved hook as I have heard that scuds swim with a straight profile rather than a curved one when they are at rest.
Here is the scud fly I use (I use clear glass seed beads for the beadback and tan Antron or nylon yarn for the legs):
Rainy's Beadback Scud