I'm not sure I'm ready to take the plunge to buying a propane burner for my first attempts at brewing. Assuming I can physically fit the kettle on my stove, how large of a batch could I make on a 17,000 BTU burner?
You won't know till you try, that is the real answer. Just because it it advertised as 17000 btu does not mean you have it at your pressure. I would buy the kettle and start putting water in it. this is assuming it will fit.
I don't know how many BTUs my stove is but I have a fairly new gas stove with 4 burners.
It can do a 5 gallon brew if I position the kettle over 2 burners. This heats up in an acceptable timeframe but I wouldn't try a bigger batch size.
If your kettle is small and only covers one burner that would limit things further.
My understanding from reading around is that not everyone can do 5 gallons full boil on their kitchen stove. If you buy a 10 gallon kettle and you find the stove is no good for 5 gallons you can use the same kettle for a 3 gallon brew, no money wasted.
Some people also use an electric heat stick to supplement the stove power.
My stove has an 18,000 btu burner. Takes longer than I'd like to get five gallons to a boil, and I wouldn't mind it being a more vigorous boil...but it does the job. I don't think I'd ever try a larger batch with it.
But as has already been said, you won't know till you try.
I do 4 gallon batches on a gas stove. (just over 4.5 gallons) It's a pretty anemic boil even on the high-output burner. An electric stove would probably do better because less heat is lost going up and around the sides of the pot.
I use an electric bucket to heat the mash water and sparge water. It will boil 4 gallons of water in just a few minutes; really speeds up the process.
I would start with a 3 gallon recipe in your 4 gallon pot. (you can scale a 5 gallon recipe to fit) If your stove boils it with ease, then you can think about stepping up.