Oddjob,
this photo may give you an idea how big the roe sacs should be. I tied these up for steelhead fishing in the winter time.
Normally when fishing for coho, I tend to stick to piece of roe. The only times when I've used roe sacs in the past are fishing faster water, since roe sacs have a bigger tendency to sink than roe chunks.
You can also use pieces of roe to tie sacs. I sometimes tie those up the night before heading out. The disadvantage is, it can get rather messy if you're using pieces instead of single eggs.
Preference of colour is up to you, or the fish. I really like my procured double hot red and have good success in the past. The eggs in the above photos are natural.
I hope this helps, good luck.