Although the Picasse was originally made to be fished very large, 2- 3/0, it evolved into a good small fly as well. I like to tie and use a single hook version of the fly as I think that most Spey type flies look and fish better on a single, however, that is just my personal opinion and I have absolutely no basis to go on other than what looks good to me!
The key to the Picasse is the spey hackle, which is usually Heron but can be substituted with other materials but the flies just do not end up looking the same. I use the very delicate hackles in natural black. These feathers flow incredibly well in the water.
Here is are the sizes I use and when I use them:
June - 1/0 2/0 even 3/0 if the water is super high. In lower water levels I will use a 2 or 4.
July - 2-4-6
September 6-4-2 early September and later I go back to the big uglies 1/0 and 2/0.
By the way, the Picasse looks really kick ass
when tied on an Alec Jackson hook. Especially a silver one!!!
Remember, whenever it gets really cloudy or starts to rain, the first go to fly for the Gaspe is the Picasse! Well, at least it is MY go to fly. Then again, there are a ton of other good patterns to use so all I can say is fish what you have confidence in. After all it is not really the fly that makes the difference but the swing. Then again everyone knows that already - don't they??