How to generate geojson from image?

In the following example:

It shows simple objects being generated from GeoJSON(am I using this term correctly?).

For example, this

F1 M0 0 L50,0 50,90 0,90 0,0 x M0,7 L50,7 x M 0,30 50,30 z

Generates the twin bed image

If I want to have other objects, say a computer monitor, or a CPU casing drawn, how do I get the GeoJSON coordinates?

Is there a program to use that allows me to draw the image I want, or even better, generate the GeoJSON from an existing image?

How do I make the size smaller of all the objects in the palette?

That is not any kind of JSON – it is a geometry path string.

SVG uses a subset of this syntax, as do several other graphics systems.
So it should be relatively easy to get such geometry path strings from any drawing tool.
You can also find SVG for a lot of existing artwork.

Or you can just write it yourself.

If you want to make everything in a diagram look smaller, just set its initialScale to a value that is less than 1.