I have a student this week who thinks a 16x20 canvas is small. He paints canvases that are in the "feet" range. Yesterday, he painted a 3'x3', and as I write this, he is still on-location working on a 3'x4'. It takes him five or six hours. Here's a picture of him painting at Red Rock Crossing.
I don't think I'll go quite that large. Some of the issues with "supersizing" your work include: finding a palette big enough for mixing, storing wet paintings and, of course, shipping wet paintings. For me, a 12x16 or 16x20 is plenty big.
My other student is also painting big paintings. The last time he workshopped with me, he used a little, 6x8 box made by artist Ben Haggett. (Ben makes really nice boxes that employ industrial-strength magnets to keep the boxes closed during travel. Visit his site at www.allaprimapochade.com.) This time, he's working with a 12x16 Haggett box.
I've made four paintings this week: two 12x16s and a 12x24. I'm very pleased with the extra room these sizes give me to work in. The paintings are pictured below.