The graph used by scientists that shows stars grouped by size is called a Hertzsprung-Russell diagram (H-R diagram).

Here's why:

* Hertzsprung-Russell Diagram: This diagram plots the absolute magnitude (luminosity) of stars against their spectral type (temperature) or color.

* Size: While the diagram doesn't directly plot size, the relationship between temperature, luminosity, and size of stars is well established. Hotter, brighter stars tend to be larger, while cooler, dimmer stars are smaller.

Therefore, by analyzing the position of a star on an H-R diagram, scientists can infer its size and other properties.