I've never understood why people brew 5 gallons when you could brew 30 gallons in just about the same amount of time with the same amount of work. 2 cases of beer just ain't going to cut it...
Seriously though,
1 gallon is an amazing way to start. Virtually no investment in equipment, most likely the stock pots you own will get the job done. And everything else you need (hydrometer, airlock, bottling equipment, etc) you will still use if and when you go to the larger batch sizes.
I did 1 gallon for years, literally 100s of batches. No regrets. I only went to 3.5 gallons recently. I simply do not want to drink the same beer over and over and over again. I also like to brew indoors and don't want my brewing equipment to take up more space than it already does.
1 gallon AG is a great way to get in with little stress and little cost. Learn the ropes, find a few recipes you really like. Make sure you really like brewing. Then you can upgrade to whatever you want.