Scale can make or break your illusion of 3d space. It’s the correct sizing of objects within the linear perspective of your scene. Proportion a subset of scale: it’s the size relationship of component parts within each object. If one character is half-sized, or a doorway is doubly tall - the entire image loses credibility.
Whether it’s surface details or object size, scale and proportion challenge beginners. If you’re focusing on one object at a time, it’s easy to lose track of the scene in its entirety, and scale problems are common. This episode of the fix list explores issues with scale, and how to avoid them.
As an additional note, scale is almost always tied to linear perspective - so make sure you’ve learned the basics.
Principles of Design: Scale and Proportion