I have been doing full boil five gallon extract batches, and I believe my boil kettle is around 7 gallon (I started with 6 gal. of water last weekend and still had a little head room left).
Bob,
The "next step" is only as far as you want it to be. You can certainly continue with your 7 gallon kettle and AG 5 gal. BIAB with a $2 paint strainer bag, yet a larger kettle and a "better" poly voile bag will make the process easier and perhaps more enjoyable. As you probably figured out, a small kettle is a tad inconvenient, but lots of people brew 5 gallons in a 7 gallon pot. With BIAB, a small pot is a slight inconvenience, but if you add a sparge step you can even brew very large beers! Hell, with some cheating one can brew 5 gallons AG in a 5 gallon pot....it can be done, perhaps not ideal and slightly inconvenient
There are a few small pot "work arounds" that can be done....
1. sparging, either dunking the bag in a separate bucket, or pouring sparge water gently over the grain bag while held over the kettle
2. reserving some of the sparged wort and adding it back to the kettle as you boil off a bit....note all the wort must be boiled for at least 20 minutes...
3. topping up the fermenter with a bit of water, while this will decrease efficiency a bit...it is a viable option!
4. kettle valves while a convenience, also require cleaning....not mandatory at all IMHO
5. kettle mounted thermometers are for show IMHO, a basic hand held digital stick is all you really need, again JMO
Don't feel you NEED a 15 gallon kettle to BIAB a 5 gallon batch....my advice is to let your brewery grow in the direction you feel comfortable. Nothing wrong with running what you have and gaining a little experience....
A talented brewer could brew in his hat....just an old joke I favor....cheers!
wilser