I really only use my drypack in the winter so I am looking at it from a winter safety angle, I like bright colors that can be seen.
A nice bright orange is my color of choice. This is the current color of my drybag for my pack. However, I know from other posts that many users on this forum will want a green/brown so that they can blend in a bit.
As for size, 50L is perfect for me. I have a 65L drybag aswell and it sees very little use. I think the only time I have used it is when I am camping with my girlfriend and I have to bring bacon, eggs, two tents, bottles of wine, microwave....
Also, I am a fan of the thinner and taller bags than wide ones. I notice that my 65L bag is very wide and creates a wider footprint on the trail that can become a problem if things get tight. Not sure if you are looking for this type of info but I want to make a cuben one and these are all the things I am considering.