I start with 5 gal carboys; despite the rather negative commentary I received on this site. Let me explain! The very first kit I ever made (Selection Estates- old vine zin), I followed exact instructions and topped primary bucket with water to US gal mark; only to end up with a Super Low 1.080 starting SG! Talk about a watered down Zin in the making. Here's my rationale now: I don't focus on the total volume of wine when starting (5gal/6gal), but rather a set range for my starting SG so it will be more true to the wine and the kit contents. So with my red kits it's 1.095-1.100 and whites 1.090-1.095. I add small amounts of distilled H2O, stir, and then measure for the SG. In most cases I yield a little over 5 gallons, but never close to 6! After primary I rack to 5 gal carboy and rack excess into spare wine bottles that all get topped with airlock. Throughout clarifying and stabilizing and aging process I top the 5gal carboy with the wine from the kit in my spare wine bottles; so end result will be 100% true to kit contents; with no outside influence of outside store bought wine.
I'm almost adhering to that method, but measuring SG 12 hrs after adding grapepaks and pitching yeast is bit tricky to adding more water. I generally come up short after transferring from primary to a better bottle anyhow, and have been flirting with not topping up during the 10 day secondary fermentation. I can then rack back to primary, stabilize and rack to 5 gal glass + 3 Lt and dividing up the added oak cubes. I'm now trying not using finings and waiting to rack again at 2/3 months.