ACEO-size artwork is a wonderful size to work in.  Always 2.5 inches by 3.5 inches, the uniform size can be used to depict either simple designs or complicated drawings.  A very portable size to work in as well.  I have completed oil paintings, watercolors, gouache paintings, and of course pen and ink drawings in this miniature format.

ACEO "A Simple Cherry" Prismacolor colored