Working in a smaller format has many advantages for plein air painters. The first consideration for many plein air painters is getting the canvas to the location where the painting will be created and back while still wet successfully. To solve this very first problem painters us everything from professional wet painting carriers to clean pizza boxes. Along with a smaller canvas, packing your supplies will need to be packaged smaller as well. Pochade boxes solve this problem by giving the artist a place for mediums, paints, brushes, turp and of course the palette. There are many companies that sell these boxes, but you can make your own from small wooden boxes you can buy at craft stores. Whichever you choose, the main thing is to keep it ready to go. Some even have more than one so they can keep one in the car or take care of a different medium.
Once you get to the location and begin painting you will find that working in a small format is the best way to break painting down to its essentials because it forces you to focus on what is important and leave out unnessary detail. Realizing that you only have a matter of two hours to capture the light the way you see it brings home the point of working with economy of brush strokes. It's much like the carpenter rule that says measure twice cut once. If you are working with a smaller canvas then you can concentrate on making decisions just for that size and zero in on what drew you to the site in the first place instead of having lots of room on a larger canvas to expand and add more to your first vision.
When you paint on a small canvas, paint using brushes no smaller than a #8 bristle brush or try your hand using a palette knife. It's a great way to learn how to handle a brush properly and with that economy of brush stroke as mentioned before.
Many plein air painters use their paintings as studies for excuting larger works in the studio. They use their smaller works to work out compostion, color, vlaue and design issues. Working small also gives freedom from thinking that you might be wasting supplies, time or energy. It allows to to try out different techniques, like bush calligraphy, different paint thickness, different color combinations which is a great learning/teaching tool to use in workshops or just independent study. If you don't like what you have done, it's easier to toss than a larger canvas for sure.