I want to know some more details about "Texture resolution" setting in the "Terrain" section of P3D options.
This setting is given in centimetres.

So what does it mean if I will set this to e.g. 30 cm? What is the scale factor for that?

In other words - let's assume that my 737 PFD has 15 cm of width.
Is that mean that if I supposedly "take" my PFD and put it on a ground my textures will be as big as two PFDs (2x 15 cm = 30 cm) ?

I don't know what scale I should use to understand that. 

Could you please clarify?

This might help.

Texture size and resolution are two different things. A texture can be any size (although normally a power of 2 in our sim, a common size being 1024x1024) and the resolution represents the size of a single pixel.

So 30cm means that a single pixel covers 30x30 cm of the ground. As each 30x30cm is represented by one pixel, it can only be a single colour/tone. So if you took your PFD outside and laid it on the ground just as a satellite flew overhead and took a picture at 30cm resolution, you couldn't expect to see it, as it's total size is less than a single pixel. At 7cm (our highest setting, I think) an area of 15cm square could be represented by 4 pixels at most, so don't expect to be able to read it... Your 3x3 metre garden shed roof would be represented by 10x10=100 pixels.

A texture at 30 cm, if it was 1024x1024 pixels, would cover an area of 307.2 x 307.2 metres.