How can you measure the size of objects with a smartphone for the application to work with this data?

Question

Android

For about 5 years, the idea of ​​a fun application, but very utilitarian, for iOS and Android has been lying around.
The only gag is which method to use to measure objects and pass data to the application.
I heard about separate developments in this direction in order to sell clothes, however, I am more prosaic - I want to make an application for myself, and only then how will it turn out.
A specific example is to measure a balloon. The option to put a plastic card next to it and take a picture does not simplify the decision ...
I will be glad for any ideas.

Answer 1

A is the camera of the device,
BC is the height of the object we want to measure.
What do we know? Only a 90 degree angle, everything else is unknown, additional methods will have to be introduced.
PS 2 cameras appear on some devices, it will be possible to roughly estimate the size (only how to process it?).

Answer 2

it is usually proposed to put a plastic card next to it, because it is a standardized size, and calculate by comparing the sizes.
but I want to be more beautiful - there is an idea to take one step from a distance and take it at close range, calculate based on the difference, with a tolerance for the difference in the average individual's step length. but the clearance is too big.