Consider how aspect ratio can visually skew line charts
The aspect ratio (i.e., the ratio of the width to the height) of a chart, usually a line chart, can drastically affect how the data is perceived. In other words, the line on a square chart will have a more drastic slope than a longer rectangle chart. It’s been suggested that a slope of 45 degrees is optimal, but this isn’t always the case.
The important thing is that the chart’s aspect ratio doesn’t dramatically exaggerate (or understate) how changes in the data are perceived.
Ultimately, the shape and size of a chart should be determined by the data it’s representing. Again, it’s the designers responsibility to take into account how a poorly sized chart can misrepresent data.