It is most important to get it right on the drawing before starting to paint. I was always taught the importance of drawing and planning first. Sometimes I even paint a smaller version of the final larger painting to iron out the wrinkles before painting the final large painting. Whether it's oils, acrylics or watercolors, planning and drawing pays off.
From a photograph or print I usually scale from it to the work I want to do. In other words let's say I want the painting three times the size of the photo or print I am working from. I measure using a set of pointers placed on the photo and walking them by three on my paper or canvas. I do this every time I draw whatever is making up the drawing.
Another way to do it is to draw squares or a grid over the photo or print and on your drawing or canvas draw the same grid three times larger. Copy the lines between the squares and you are almost guaranteed an accurate reproduction whether it's a portrait or any subject. I have done portraits using this system. I was taught this by an British portrait artist back in the '50s. It's still good today.
Still another way to get it right is to measure with your thumb. Transfer the distance between objects by holding out your arm full length and measuring with your thumb how large it is. Then mark it on your drawing or canvas. This system is not as accurate and requires more skill.
A final method other than just drawing what you see without measuring is to project the image using a projector. This used to be called cheating. However, if you are already an accomplished artist and you know how to draw it anyway, you can save yourself a lot of time by just using the projector. Many artists use them. When you come to paint you usually lose the drawing anyway, so skill is still needed to finish the work.
