I would have thought that a volume of 20L would be a reasonable sort of size. 1 mole of gas at STP occupies 22.4 Litres. For helium that's 4 grams.
The capacity of carbon dioxide "sparklet" bulbs varies but, some readily available ones hold 8g or about 0.18 of a mole or 4 litres at STP. So if you had the same pressurization you would need a canister about four times the size of a sparklet bulb. Sounds very feasible to me.
Might be problematic in really heavy weather. Might be issues with it deflating over time as helium tends to leak pretty easily too. But as with all safety gear there are pros and cons. This could have a place. Weight for weight if I had to pick one solution, it would be an EPIRB/PLB with a strobe. I don't know too many canny paddlers who only carry one option. Marine radio, flares, PLB, signal mirror, sea dye, V sheet. Most paddlers have room for more than one of these.
If I was going to carry a balloon, I would want to incorporate an extended antenna into the tether. It would boost the range of a marine radio dramatically.