I've been thinking about this in the last day or so. I'm not an expert on throwbags and some folks on here know a lot more but here's my two cents before I turn in...
All throw bags are not created equal. Some have stronger rope than others, you want to make sure the rope floats and is supple static line. I think the bags DRE sells are about 5/8" and rated to about 1000 lbs. Is that right Matty?
Kayakers usually carry smaller diameter line (3/8"?) and about 50' ropes because of the compact size. Us rafters usually have 75' ropes (5/8"?) in bigger bags. The way I learned to stuff a throw rope is to have the rope coming from behind you and running over your shoulder so the empty bag's in front of you. This way you're stuffing the rope into the bag about a foot at a time by pulling it down over your shoulder, making sure the rope is layered in the bottom of the space available in the bag so it plays back out continuously. This is about the fastest way I know and I've never had a bag knot when deploying. If you need to throw again quickly, you can do so without restuffing the line by filling the bag with water to give it enough weight for the throw. 75 feet is really about as far as I can throw a dry throwbag so that's optimal to me.
I feel like the longer the better because not only may the swimmer more than 50' or need the extra time floating by to get the bag securely in hand. If you use a pully for mechanical advantage, you won't have much to work with once a 50' rope's doubled over.
Don't let your first throw be to someone that really needs it. Do dryland practice throwing with a buddy walking across in front of me about 50 feet away so you can get the hang of getting the bag ready, leading the "swimmer" and throwing accurately without consequences if I botched the toss. Every practice throw is followed by practice rope stuffing. Make sure you're securely braced before you throw, especially if you're going to haul in a runaway raft. Be prepared to let go or cut the rope, don't wrap it around your wrist - thnk of the force and momentum of a swiftly moving swimmer or loaded raft. Not only are there plenty of stories of people missing the swimmer or getting pulled into the water when the slack runs out but I've also heard of people throwing the entire bag - oops...
I've had some perfect throws the swimmer caught and I felt like a rockstar and the next time had a spastic-looking useless throw because I was too hasty and forced it. Practice beforehand, and when the time comes, keep cool, make sure you're set up properly and in a good stance ready to brace or hand the rope off to your passenger while you jump back on the oars. Be judicious when deciding whether to throw a rope. The more ropes out and in the water, the more entanglement hazards that can drown someone. I've found myself under a flipped paddle boat with loose rope all over me going through rapids - yikes!
Which reminds me, always get the deployed rope back into the bag ASAP so it's not an entanglement hazard. Practice throwing with your boating partners a few times a season so the rope is also inspected periodically and you can re-stuff it properly from time to time to prevent / eliminate any random knots that may develop as the rope is being jostled in the bag over the season.
I've got a 'biner on a loop at the end of the rope in the bottom of the throwbag in case I throw to someone that needs to clip onto a raft quickly. Ideally a caribiner on the throwbag should be on the outside of the bottom of the bag and in a labeled pocket with a Velcro tab closure so it can be accessed easily and also so it doesn't knock a swimmer's tooth out if one manages that bullseye toss - something to consider for some custom stitching. Maybe someone with a heavy duty sewing machine could sew something like that or create a mesh bag that'll allow the rope to dry easily in the bag? Preferably a retired guy with lots of time on his hands...
Your bowline may become your main rescue line, make sure it's up to the task. I've usually retired old throwbags to the bowline after a few years, now rethinking this practice because...
When my boat was upside-down the other day, my bowline was the workhorse rope because my throw bag was sitting lashed on top of the cooler where it's really handy if the boat's right side up and I'm in the captain's chair. That wan't very helpful there when I was standing on the bottom of my boat and I sure didn't feel like going back under to get it. I'm thinking of strapping my main throwbag to the outside of the frame just behind the oar tower so I can get to it easily from inside or outside the boat, looping a 3' strap around the frame a few times so the strap stays on the boat after it's unfastened, and I can get the throwbag off easily from the bottom of an upside-down raft. I'd like to know where other folks put theirs so they'll be easy to get to when the boat's upside down.
I might get a bag to wear on my waist as well since I may not be lucky enough to be boating the kind of great folks I was with recently.