I have done 10 gallon all-grain batches for years. Lately I have been doing 5 gallon partial mash batches because of both time constraints and as a way to test out new ingredients and formulate new recipes that I don't feel like brewing 10 gallons of if I don't like them. To the last point I have been thinking about going even smaller like 2.5 gallons and brewing a few batches of the same thing and only changing 1 or 2 variables in each one.Still, I have more fun going big! (when I have the time)
I wrote about it last year, but the thing that I did to solve my "not enough variety" complex was to do funny things to the different batches of wort. Think multiple yeast pitches on a much, much bigger scale.