Screen size is most meaningful when attached to a page you want to read. A novel with adjustable text, a comic, a technical drawing and a scanned letter do not use the available area in the same way. Gather examples of the material you read most often before comparing diagonal measurements.
For supported ebooks, Kobo's reading controls provide an example of adjustable font size, margins and line spacing. Its PDF controls instead include magnification and navigation around a fixed page. That difference explains why a compact reader can feel comfortable with one file and cramped with another, even though both are technically supported.
Compare at your own text size
Set a sample book to a size you actually want to read. Count roughly how much of a paragraph remains visible and try turning pages several times. A larger display can show more content, but whether that helps depends on your reading style. Do not shrink the font merely to make a small device appear equivalent during a demonstration.
For a document with columns, follow the reading order across a full page. Check a footnote and return to the main text. Notice how often you need to zoom or reposition. The inconvenience of that repeated action matters more than whether the smallest label can eventually be made legible.
Measure the device's whole body and proposed case. The screen diagonal does not tell you whether it fits a coat pocket or how much space it occupies beside a notebook. Cut a piece of cardboard to the listed outer dimensions if you cannot handle the device in person; this is a size comparison, not a substitute for testing the display.
Decide where the reader will rest
A larger device used mainly on a table can have a different appeal from one held during a standing commute. Check case weight, grip area and supported stands. If you plan to read with one hand, try reaching page controls without repeatedly shifting the device.
Choose the size around the dominant job. It can be reasonable to use a compact reader for novels and retain another screen for occasional complex documents. Conversely, if reports are the main reason for buying, assess them first rather than letting an attractive novel demonstration make the decision for you.
