I have to say that I was really impressed by this app. It is beautifully designed, simple to use and chalked full of data.
As far as relative sizes goes, the app took me down all the way to Planck length, which shows it as the smallest unit of length and in the billionths of a yoctometer – a yoctometer is equal to 1 quadrillionth of a nanometer, if you can get your wits around that! All are defined. Pictures of particles stop at 100 yoctometers and to celestial objects that are in our galaxy and beyond. That's quite a view!
As you span through the relative sizes of different object, you can tap on them and get a brief description with audio narration. There is additional data that you can read. On this same page, one can tap an explore button and compare this to other objects.
Did you know that the world's smallest snowman is 25 micrometers? In comparison a human embryo, of three days old, is 5.2 larger, and a flea is 80 times larger. I sure didn't know that the Eiffel Tower is equal to about 2.3 of the Great Pyramid of Giza. It shows you this – right in the app!
In the settings, there is the option to turn of the music, change text size, change the narration voice per country, as well as choose from metric, imperial, or scientific unit systems.
All-in-all, this is a great tool for giving conceptual understanding of size and distance, as well as comparisons of two different objects and interesting facts!