The standard American whiskey barrel holds 53 gallons. Why the odd number? Because that works out to almost exactly 200 liters.
These are the barrels all of the big American whiskey distilleries use.
Today, more and more micro-distilleries are making whiskey and most of them are using smaller barrels. Barrels can be made in literally any size you want, but some of the standard smaller sizes are 5 gallons, 10 gallons, 15 gallons, 20 gallons and 30 gallons. It isn't just distilleries. Wineries use barrels too and some of them prefer the smaller barrels as well.
The cooperages claim smaller barrels don't make a difference, but that's both counterintuitive and counter to most anecdotal evidence.
Size isn't the only consideration. Other variables include source of the wood, how and for how long it was seasoned, how heavily it's charred, even how thick the staves are. I'm told that some of the very small 'presentation' barrels that some people are using for aging have very thin staves (the dimension from the inner surface to the outer surface, the depth not the width) that may not be able to withstand high temperatures. They literally pop when it gets really hot.
In Texas, Garrison Brothers uses 10 gallon barrels primarily, but they're experimenting with 20 and 30 gallons as well. Finger Lakes in Upstate New York uses 'quarter casks,' which I assume are in the neighborhood of 10-15 gallons.
Barrels Unlimited is a company that sells barrels in a wide range of sizes and you can order them directly from their web site. Note that the 5 gallon barrel is $210 but the 30 gallon barrel is $235. If you're trying to do something commercially, you have to consider that the barrel-cost-per-gallon can range from $8 with a 30 gallon barrel up to $42 with a five gallon barrel. Like everything, the advantages of smaller barrel aging come at a price.
The reality is that three things happen in the barrel that affect the whiskey: evaporation, absorption, and oxidation (and some other chemical changes). Absorption happens pretty quickly and can be accelerated by using smaller barrels. Being in a warmer climate also accelerates absorption. Evaporation tends to concentrate flavors and I'm not sure what effect, if any, a smaller barrel or warmer climate has on that, but I know humidity is also part of that equation. Oxidation, which tends to round things out, is strictly a function of time.
So what? A lot of mirco-distilleries are afraid to make whiskey because they find the 4- to 10-years (or more) that most American whiskeys are aged overwhelming, but it's starting to look like some pretty tasty tipples can be made in two years or less using small barrels.
Some people will say things like, "we aged it for 18 months in a ten gallon barrel and it tastes like a five or six year old whiskey." It's not really about finding a faster way to make Jim Beam. It's about crafting something original and tasty, and marketable. Smaller barrels appear to be part of the answer.