The size choice definitely depends on the size of the river and the strength of flow of the river. On small light rivers a small drennan will do fine. On the Thompson I like larger Drennans which can support lots of weight to get down and dirty in the heavy flow. On the average, the 11 and 20 grams are fine.
I've been using Drennans for years, acquiring them from Brian at West Coast years before any of the other tackle stores were carrying them. Here's a couple of tips I can pass on which may help extend the life of those pricey jewels:
If you crack your beauty on a mean rock do not throw it out. Make sure you completely dry the bugger out and then carefully use Aqua Seal to repair the cracks or holes. A little bit of practice and you won't even be able to tell the Aqua Seal is even there. I have repaired these floats and kept using them for quite some time after wards.
Tip 2: A good way to keep from loosing the Drennan after breaking off your main line on a snag is to use 4 float stoppers under the Drennan. Don't put them all together, put 2 directly under the float and then the other 2 staggered about 3 inches apart closer to your swivel. If you are stretching the crap out of your main line while trying to free the snag and the line breaks causing a big elastic SNAP BACK; the Drennan oftens quickly slides off of the main line due to the momentum of the SNAP BACK. With the four stoppers in place you now will have several "blockades" to keep that precious float on your main line. Countless times I have snapped off and still thankfully had my Drennan still attached to the main line. Most of the time it is the very last, bottom float stopper which finally halts the float from finding freedom in the river. Also it is good practice to use the very tightest float stoppers as possible. If my main line is 15lb test, I will squeeze on float stoppers labelled for 8 to 10 lb line.
Tight lines.