If you have drawn a small sketch and want the painting to be larger, or if you have a small photo and want to make a larger sketch to paint, then you should learn how to do a graph or grid for this purpose. You can find videos and explanations of how to do this on the internet. I will tell you the basics. Always start by finding the center. Find the middle from the long and short sides and draw a straight line both ways on the photo or sketch. Where they cross will be the center. Precisely measure one inch from these lines and add more lines in a checkerboard fashion, being sure to always double check the measurements. Now decide how big you want your final painting to be and make the distance between the lines increase by the correct amount. For instance, if your sketch is 8×10 and you want your painting to be 16×20, then the distance from one line to the other will be two inches instead of the one inch you started with. You can see that I also connect the diagonals too.

Now look at your original photo or sketch. Starting with any feature, draw on the larger graph a mark in a square or triangle that exactly matches the smaller one, just remember to make it the increased size. If you are careful to follow the pattern and not confuse the placement of your marks, then you proportions and likeness should be good.