MOD: Perhaps this thread should be somewhere else. If so, please move it 
As a user of Cycles with the GPU, you always have to compromise between eye candy and VRAM usage. Even though I have tried, it is really hard for someone in a normal desktop environment and a simple program like GPU-Z to try to understand what it is that actually uses up the VRAM. And since I am no programmer, I was hoping someone with the understanding of Cycles or path tracers in general could at give us a basic understanding for how much space things actually take up And, if possible, we could create a list of what each feature consume so it will be easier to plan your scenes in the future; to already know, depending on you amount of VRAM, how many polygons you can have, or how many textures of a certain resolution.
Polygons - Does every single polygon take the same amount of space? If I have 100 000 polygons, will that take up the same amount of VRAM regardless of structure and placement?
Hair - Is hair just normal polygons to the GPU? Could you could take the amount of polygons per strand, multiply it by the complete number of strands to get the number of polygons, and the the space required? Does children affect this in any way or are they saved in the same way?
Textures - How much does each pixel take up? What is the difference between using a 1024x1024 texture and one with 2048x2048? Do the number of textures affect or is it the total resolution that matters?
Settings - Are there any render settings that in some way makes the calculations extra hard or big, and therefor uses up more VRAM? The tile sizes, no caustics and all of those.
The Scene - Perhaps even certain light configurations or shaders could affect this?
Placement - Will everything in your scene get passed on to the GPU (regardless of if it is placed 1000 BU behind the camera) or is there some kind of first elimination? Would you save space by removing anything that you know will not affect the light of the final image?
Ray Visibilty - If you remove certin types of ray visibilty for objects, will that make a difference or is everything just loaded to the GPU as normal as long it is used in any way in the scene?
Rendered resolution - How does the output image affect it? If I go from 540p to 1080p, what difference will that make? (Would this perhap take the same amount of space as using a texture with the same resolution in you scene?)
If it is impossible to give general answers to this, I completely understand But I really hope it is possible! Prioritizing your scenes would be so much simpler, with much less guessing and having to do over if you just knew your “budget” and what you can get for it beforehand.
Thanks in advance!
Same level of detail everywhere is key.