Of course, we learn early on in this hobby that all liquor comes out of the still clear, and that it takes white oak and time to make a good whiskey or rum. There's a ton of information on this site and out on the net about but it can be hard to find it and sort through the good, and not so good advise.
One thing about the commercial whiskey makers, they are masters of aging and blending....They age their whiskey in new or used white oak barrels that are usually charred or toasted, and the typical aging strength is between 60 and 65% ABV. Their taster's highly experienced noses and taste buds are extremely prized and we can only assume paid well for being that good. They surely have some sort of sampling schedule that's used to make sure it comes out just right, barrel after barrel.
We, on the other hand might have extensive experience drinking whiskey, , but no real idea how to age it.
It's safe to say that you'd be hard pressed to go wrong by copying what the pros do. A full sized whiskey barrel holds approximately 55 gallons and has an interior area of 3200 sq in. I don't know about the rest of yall, but I need to scale that back a bit. There's no place around my house to keep a mess of full sized barrels. If you divide that out, it equals 57 sq in / gallon...or approx 15 sq in/quart.
American white oak is what you want. It should be air-dried for at least a year, and you should only be using the core wood, nothing near the bark layer. I usually cut my oak slabs so they're about 6 square inches on each side (2x3, 1.5x4, 1x6, you get the idea) ....I like to leave em a little thick so when I char it with the MAP torch on the outside, the core of the wood doesn't get super hot ...I don't burn 'em so much that the entire outside looks like a burning coal ...just until they're all black, and start on fire at the edges, and then let 'em go out and drop 'em in some clean water. More often than not, I'm aging in quart or half gallon mason jars. Drop the required amount of oak in the container and fill 'er up. Place in a safe place and keep your mitts off. I opened a jar of UJSSM recently that had 2 of the usual sized sticks in there for 2 years, the stuff is heavenly...wouldn't change a thing. It tastes like a good commercial whiskey yet smoother.
I've gotten great results with a small barrel as well. A 5L sized barrel holds approx 1.32 gallons and has a interior area of 350 sq in. That equals 266 sq in / gallon. Thats about 4.5 times the area (per gallon) of the full sized whiskey barrel. So you could expect the whiskey to mature MUCH faster in a small barrel like that. I think the result you get from a small barrel will depend a lot on the quality of the wood in the barrel …. the barrel I used was not an expensive barrel, and it had a light char. The Carolina Bourbon that I used the barrel for was 3 months in the barrel and turned out excellent, but I gotta wonder if it could have stayed in there longer. It never had a woody, splinters taste and turned out with tastes of vanilla, cherry, caramel, and that mellow spicy charred oak taste that we like whiskey for. Keep in mind that the small barrels lose a little more to the angels than the big barrels.
The chart below should give you a good idea of the kinds of flavors and smells that you can coax out of your oak depending on the temp and level of toast/char you have. That 350-400 area looks pretty inviting, eh?
I'm not a big fan of huge temperature extremes (freezer in day to counter at night) but I do agree with temperature changes, like the seasons (outdoors). I tend to think the heat of the summer may bring more flavors out of the wood , and the cooler weather in the winter mellows the spirit ...least I'd like the think so. So age your hooch out in the barn/shed , just keep it away from prying eyes, kids, rodents, etc.
In the end, I think the most important things to pay attention to are :
Dilute your spirits to 60-65% abv
Use only White oak.
Toast or char (or both) according to taste,
and go with something in the range of 50-100 sq inches/ gallon .
Leave it as long as you can keep your fingers out of it.
If in doubt, (like with a small barrel) sample every two weeks .