6x12 and 7x14 trays are practical sizes for many purposes. I guess you go from the cell to the final plant pot, not into the ground?
>> I typically only start a dozen of a particular plant
I don't think I ever start an entire 11"x21" tray of all-one-plant! I always cut my prop trays up into 4-5 chunks or slices so they are easier to swap around as some plants emerge or need potting up before others. Like you, I start a few rows of each thing. I label one cell in each row with a mini-blind slat cut in half the skinny way. Those slats also hold plastic film up off the soil surface.
Although I have started a whole tray of the genus Salvia: a few rows of each species or cultivar.
