Speaking for the UV layout itself, the UV coordinates are store in the
0~1range, so it's actually a percentage of the image, I think.
That means they should stretch seamlessly to adapt to any image size, even if the proportions are not the same and the image with to height ration is different.
Bottom line is, for a certain UV layout the actual size of the texture in pixels should not matter much.
As for the size of the texture itself, if you want to maintain the same relative quality level for each object, then yes, you should scale it proportionally. However your math is slightly off there, you have to account for image area, not image size alone.
3 x 2048 x 2048 will give you a 6144 x 2048 texture, not a
6144 x 6144.
Now have in mind that that is a really large texture, you may find yourself hitting the memory and performance constraints of your computer.
This should be fine for 'offline' rendering, but for real time rendering, like Blender Viewport, Game Engines, OpenGL or DirectX mobile games, etc. , that could be more of a problem, especially having in mind older hardware.
Also as Gliderman mentioned, computer hardware generally likes exact powers of two sized textures, so it would probably be wise to use a
4096 x 4096texture instead, and use the left-over square area as a tile for an additional object you would add to your existing texture atlas.