If you're adding extract as part of your fermentables in addition to mashing grain, it's considered a
partial mash or
mini-mash.
For a 5-5.5 gallon all grain BIAB batch you'd need
at least a 10 gallon kettle, since it got to hold the full mash volume including the bag with grain.
Incorporating a sparge in a separate vessel (large bucket) after you lift the bag out, can help with managing the volume in a kettle that's a little too small, while adding a few points to your mash efficiency. Topping up during the hour-long boil with some leftover wort from the sparge, can also help to keep a comfortable headspace and prevent boil overs.
Is that what you actually want to achieve, a larger kettle, not necessarily larger batches?
I often brew two 5.5 gallon batches back to back. One prep and one clean up.
Since they can be entirely different beers that way, it's the most flexible.
I don't brew many 10 gallon batches anymore, I prefer to have the luxury of variety.
Now when I do decoctions, I haul out the big 15 gallon kettle. 3 1/2 hour mashes aren't worth it for only 5 gallons, IMO.