The format is the outer boundary or edge of where you will draw or paint AND the space inside this boundary. Put another way, the format is the edge of the composition and all the space inside the edges. It is where the composition will be drawn, painted, or printed. For example, if you have an 8.5 X 11 inch piece of paper and you use the whole paper to draw or paint on -- then your format would be 8.5 X 11 inches (the whole paper). It could be used vertically or horizontally.
If you were to draw a new rectangle, square or other shape onto and within 8.5 X 11 inch paper and compose inside of that -- then this new smaller format could be any custom shape (square, rectangular, or other) that you want to create.
If you have a prepared canvas that you made or bought -- no matter what size the canvas is -- the whole canvas is your format.
In order to make compositional sketches (sometimes called thumbnail sketches) before you begin to paint on the canvas, you could make small scaled down formats for your compositional sketches to help plan your painting's composition. Again, these are small scaled down versions of the bigger canvas (height X width measurements). Both the canvas and the smaller drawn formats would be of the SAME RATIO (height X width) . Specifically, if the canvas measured 18 X 24 inches then you could make scaled down versions of this with scaled down compositional formats measuring 3 X 4 inches (both 18 and 24 divide by six). Use compositional sketches in small formats to make small and large drawings as well as paintings.
Tip: To make scaled down formats without using math -- I like to use an architectural scale available at most office supply, drafting supply and sometimes art stores.
Surprise: More often than you would think, beginning students take something that they want to copy that is one ratio (H x W) -- let's say square and try to copy it to an 8.5 X 11 inch format, for instance. They run into many problems as they discover that they are trying to make the square composition stretch to fit into the longer width or height of the rectangular format. Be on the lookout in your own work for this 'happening'. It is easy to do!