Most people know what an iPhone X (or similar device) feels like. Not many people have held calipers (or remember what holding them feels like) and not many people can put into context what a marking on a ruler actually means.
For a quick scale judgment, a smartphone is perfect. It's not supposed to be precise.
I respectfully disagree. Smartphones do not come in a single standard size. Using them as comparisons will end up with the same problem as using coins or banknotes--it is only helpful for those already familiar with the specific comparison item.
A "for scale" comparison is most useful with a metric-scale ruler. Metric lengths are standard worldwide. Including an inch-scale ruler may help US-based persons. The calipers were useful inasmuch as they had a metric length scale etched into them--a ruler with extra bits.
By your argument, it is acceptable to report large lengths or areas in multiples of US football fields, volumes as multiples of Olympic-size swimming pools, or database sizes in multiples of the entire literary content of the US Library of Congress. Journalists do this frequently, and every time, it drives me absolutely bonkers (an aggravation level equivalent to 3 rush-hour traffic jams).
For a quick scale judgment, a smartphone is perfect. It's not supposed to be precise.