With few exceptions we draw and paint on a rectangle which is our canvas, the term for the pictorial surface.
Merrily traipsing off to the art store and buying a pre-stretched canvas, most likely the ever popular 16 x 20" size which is on the supply list of practically every portrait painting class, immediately relegates your work amongst the minions of earnest amateurs. Alas the beginner's downfall has begun. Controversial words? Yes. Tyrants do not pull punches.
This is a tragedy whose historical routes are found with the commercial mass production of art materials.
Well, heck! You might even retort: I just want to learn how to paint. What is this nonsense about my very affordable and, I might add, convenient canvas?
I assume that if you have read this far that your intent is to acquire your foundational skills as quickly and efficiently as possible. That means killing several birds with one stone. (Apologies, of course, to all birds.) Learning is best achieved by a layered approach. This also means that that first layer must be rock solid. And that first layer is mastering the skill of accurately drawing the height/width proportion of any given rectangle by your eye alone. That means no pre-measuring. None! Sure, some people do cheat in their training -- and I know who you are -- but they are only cheating themselves.
There are good and bad rectangles. Bad rectangles are static, lifeless. A painting begun on a bad rectangle (canvas) is stillborn. The 16 x 20" canvas is a notoriously bad character. And to select such a canvas because the framing options are oh-so-convenient is too depressing to even contemplate.
Good rectangles subscribe to either the octaval (musical chords) or the dynamic (natural design law). Both proffer harmonic pictorial areas. Every natural and well designed object corresponds to a dynamic rectangle. To create drawings and paintings that are unified and harmonious requires an ingrained appreciation of the power of dynamic rectangles which is proportion.
To wit, when we draw a portrait our first step is to strike the arabesque, the overall outside shape of the head. This entails accurately gauging both the height/width proportion and the actual shape. Doing so requires training. There is no other way around it.