One thing that is often neglected when people are "doodling", but important to keep in mind, is the use of "Standard Sizes". This makes it easier to frame and hang. Here are a few sizes that are common for smaller pieces of art [given in inches and their centimeter equivalents]:
Again, try to plan out how your art fits inside the "Format" (i.e.: the size of the paper or canvas) before drawing or painting. This helps with Composition and keeps one from wasting materials by only focusing in on a small part of the available space.