We need the long suasage shape to stay upright to be visible in the swell, the short round lift bag types are not visible in UK Waves, to stay upright it needs to be full and you need to keep tension on the line or it flops over. The line also needs to be able to have extra deco tanks sent down it and message buoys sent up hence the thicker line (hopefully only if there is a problem)
1/9th (80m) of a 2m long 25cm sausage contains 1.7 litres of gas, like to bet you can hold that!!!!, certainly not on a rebreather as you cant breath out to compensate. and it gets worse as you get shallower
200/9 = 22.2
2 pie 12.5 = 79
79 x 22.2 = 1720 cc or 1.7 litres
Rough back of envelope calcs
(edited cos I cant do maths properly!!!! and I measured my blob)
At 40 m thats 3.4 litres needed.
I have reels with 50, 80 and 125m of line, I take the most appropriate
I just fill until I feel myself start to move and then let go. Normal prceedure here in the cold murk is for your buddy to clip his reel to yours, that way if yours jams (and fortunately in hasn't in the last 1000 or so times I've sent one up) you let go and you carry on sending the blob up on his reel. Or tie your reel to the wreck first so you can hold the bouy
The strong currents here mean we always deco on our blob lines hence the need for stronger line. Standard proceedure is to send a second yellow blob up the same line if we have a problem, the main blob is red. Second blob always results in extra gas coming down asap