Until pretty recently my standard brew has been 2.5 Gallons. I've done a lot of 4-5.5 gallon brews recently, but moving back towards 3 gallons these days. Since I've been moving towards lighter beers recently, larger batch sizes are fine, especially with the warming weather I tend to drink a bit more (as does my wife, plus then cookouts to share my brews).
If I have the spare time, 2.75G batch is my perfect size (keeps the head space down in my 3G carboys). When I know my time is going to be a bit more limited, I bump it up to 3.5-5G batches, because I know I might get a brew in, but not for 4-6 weeks again.
I probably average 30-35 beers a month personally consumed. Maybe 40 in the warmer months if a bit lighter brews. My wife might drink 5-10 a month, sometimes 20ish range in some months. I probably give away or serve an average of 10 a month, maybe 20 a month in the summer months. So that is around 5-7 gallons of beer consumer per month.
2.75G batches are nice, but it also means I need to brew twice a month to keep up with demand. Which I'd love to do, but isn't always practical. Of course this is also why since I started brewing about 2 years ago, I tend to go through phases where I'll brew 1-2 2.75G batches, then maybe a 3.5-5G batch over the course of 2-3 months, then I'll brew 2-5x2.75-5G batches in a weekend/week to refill my cellar and then go back to only brewing occasionally for 2-3 months as I drink down the 6-20 gallons of beer I just brewed.
As an example from mid December-March I brewed 4G of Dopplebock, 2.5G of steam beer, 5G of Rye APA, 2.5G of IPA and 2.75G of German Pilsner (roughly 16 gallons and 5 brews, about one brew every 3 weeks). That is over the course of 3 1/2 months. Then in just the last couple of weeks I've brewed 5.75G of Berliner Weisse, 5G of Belgian Single, 4G of RIS, 2.75G of English Pale Lager, 2.75G of Schwarzbier. I probably won't brew again until big brew the first weekend in May and I might not brew after that until June. Then I'll probably start picking back up a little doing at least a brew a month for June and July and then in August I'll probably brew 2-3 times and 2-3 times in September as well to get some fall beers going, little or nothing in October and probably 2-3 times in November and December to get my winter beers in.