In the beginning, when experimenting, or when studying for a larger painting, work small. Painting too large can be very frustrating until you stop thinking of Color as a specific recipe, and your shapes are all over the place and out of value key. Also, with larger paintings, the time you have invested at any point, can keep you from trying new things. Make sure that your first 50 paintings are from life, or on-location if possible, as it is very important to work on the translation from 3D to 2D as much as possible.
Time yourself to keep your mind from wandering. Try 20 minutes on 4×6” endeavors, 30 minutes on 6×8”s, that may be too long, Lol, slowpokes.