The 6" x 6" size work great for that.
I find that if I set a goal for myself with certain parameters, I tend to stick it out, and progress, at least a little. If you need a break from the intenseness that can come from painting, try mono- printing. You just can't worry to much about perfection. Surprise is the name of the game. You really can't make corrections - just do overs. And that alone teaches you lot of things. Like patience, technique, and not too take your work too seriously. You can make an image in just minutes - which I really LIKE! Fun, fun, fun! All you need is a gelatin plate, a ink roller, acrylic paint and paper. Stencils are optional because you can draw on the play and use objects as stencil.
Here is one done with two prints. First the background was a ghost print - which is the second print made from the original plate. With the gelli plate these all come out very soft and watery looking. The second color was from the first print, using a 6 x6" flower stencil from The Crafter's Worskhop. You can see from the purple edges that my home made gelatin plate is shrinking and getting pretty wonky around the edges. This won't happen with the store bought plate from Gelli Arts.
This is a photo of two prints pasted into the sketchbook. I really like the way the purple (Dioxazine Purple) looks over the olive green color.