UV Map Scaling problems

I’m trying to make hallway, single plane and add textures to it.

Problem:
I made Plane and UV map for it -> width:5000px and height 1000px
Exported it out and made graphics with Gimp.
When I open this UV map at blender it is scaled.

Question:
How to export as 1 : 1 so the pixels stays 100% accurate ?
How I can know dimensions of my plane as pixels ?

Using Cycles Render

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