How much water to boil with Kits?

You always boil as much as you can for the best results. If you can boil 4+ gallons, do it!

What you're reading about with grains is not the size of the boil- it's the amount of water to use while steeping the grains. For specialty grains, like crystal malt, the amount of water doesn't matter. But for some grains you may use, those must be mashed. Since mashing is pH dependent, a certain amount of water is used at a certain temperature for a certain amount of time. That's where the more precise water volume comes from.

Anyway, there are a couple of ways to do this easily. If you want to steep grains, and use a bigger boil, you can steep the grains in your brewpot in up to 2 quarts of water per pound of grain. Then, when you're done steeping, lift out the grainbag over the brewpot, put it in a strainer, and pour 170 degree water over that grainbag to rinse it. (That's called sparging). You can sparge up to your desired boil volume.

Another way to do it is to steep your grains in a separate little pot, while you bring the water in the brewpot up to boil. That makes it go faster, as you'll be bringing the water up to temperature during the steep, and then just add the liquid from the steeped grains into it- getting you to a boil faster.

I prefer the first method I mentioned- that way you don't have to know if you have specialty grains that can be steeped vs. specialty grains that must be mashed- and the technique is always right. Then you can easily go to partial mashing with more grain, using the same technique. It was always the easiest way for me,