There is a way of getting over this fear of doing larger pictures. Scenery, for instance, think of it as something you can see on a stage. Break it down into Background, Middle ground, and Foreground. This way your mind set could be that you are dealing with 3 subjects so you only worry about one at a time. I am sure it must be the same for portraits, vases of flowers, anything really.
I would have issues with big pastels, My silly/funny/traumatic is my family says “You have more paint on yourself than on the canvas” I wish I could figure out how to change that bad habit, laughing: