I'm in agreement that size 14 is about the most common and appropriate size. In my favorite spring creek I like to tie scuds in sizes 14, 12, and sometimes in an extended size 10. Some of the Gammarus varieties are quite large.
I also agree with Silver regarding tying most scud patterns so they are totally or mostly straight. When a scud is swimming it stretches out very straight, but often it will rest in between swimming spurts and revert back to a bit of a curved posture.
I tie my scuds on Dai Riki 125 emerger hooks. In looking at these hooks they appear to be a middle ground hook in curvature, but when the finished scud is completed the bug is more straight than curved. I like the Dai Riki 125 because of the larger gap or gape of the hook thus giving me more hooking power as I set on a fish.
I tie the size 10 scuds on a Dai Riki 280 Hopper hook.
Here are some pics of the flies I use regularly. Amber (looks like orange in the picture, but its more a yellow/gold color) is my favorite and most productive color, followed closely by a gray/olive color:
Here is a natural. Some say the orange spot is the egg sack while others claim it is a parasite living in the scud itself. Whatever it is, patterns tied with a small spot of orange works as a trigger to trout as they key in on this prey:
KG's Spring Creek Scud - Amber:
Natural amber scud:
KG's Spring Creek Scud - Olive:
Here is my fly box with scuds and mysis shrimp, along with parasol emergers and, sowbugs and damsels. You can see the colors I use as well as several of the larger size 10 scuds I use in the upper left-hand corner:
I fish a lot with scuds and have found some effective and useful patterns. The best thing for you to do is to visit your local scud waters and get some samples from the waters you will be fishing thereby matching the patterns you tie to those the fish are actually eating.
One caution: When using some dubbing brands, be careful to check the color of the dubbing after it gets wet. Many scud blends look real good in the package and you can match them exactly to the picture of a natural, but, when wet, the color darkens considerably. Take some time to experiment and find what works best.
Have fun, a scud fished correctly will bring savage strikes and strong fish like this one:
And check out the mouth full of amber scuds in this beast:
And notice the amber scud in the top of this cutty's mouth: