The wide variety of answers reflect that there are different diving conditions, and also the confusion about what the OP is looking for.
There are at least 3 basic kinds of sausages:
* Sausages designed for signaling while on the surface
* DSMB (delayed deployment surface marker buoy) that is sent up while doing drifting deco so that the boat can keep track of you.
* DSMB sent up at the end of dive as a "don't run me over" signal.
For a very compact emergency signaling device to be used if you surface downcurrent from the boat or are otherwise lost at sea I recommend the Trident FM36. Very small in a BCD pocket. I have one inside a bicycle innertube for protection and it is always in my BCD pocket (even while rinsing and drying). It is basically a huge orange garbage back you orally inflate on the surface. No moving parts. See page 194 of the large pdf file at
http://www.tridentdive.com/a111-220.pdf
I don't use large DSMBs, but there have been lots of suggestions above.
I do use the very common 4"x44" surface signal sausage as a small DSMB. I have about 30' of 3mm cord and a boltsnap wrapped around it. To deploy from 25-15' depth I simply strip off the line, letting the boltsnap take it. Then I orally inflate (or inflate in the bell of the Dive Alert LP powered whistle) and let it pop to the surface. It is the appropriate size to be used as a "don't run me over" sort of signal, but not big enough to be used for a boat to track you while drifting a long distance while doing deco. Similar to the FM35 of the above pdf. The FM35 is sold only as a surface signaling sausage and doesn't have an overpressure valve, but after watching DMs in Cozumel use this I tested one out by launching it fully inflated from 100'. The excess air just fizzes out through the nylon fabric.
This 4"x44" sausage also works pretty good as an initial surface signal. It's prime feature is that it is quick and easy to orally inflate.
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Some have commented that the bigger the sausage the better. This is true only if you actually carry the big one with you. If you choose a signal sausage so big and bulky that you leave it behind in your gear bag, it doesn't do you much good.