I ran a few tests. My CPU is an i7-7700K CPU @ 4.20GHz.
Its best time was 2’ 42" with the render size set to 64x64.
My GPU is a GTX 1060 6GB with 1280 CUDA Cores.
Its best time was 1’ 19" with the render size set to 480x270.
This was rendering the BMW27.blend file from the assets file.
Rich
For the Cpu Render mostly you get fastest results around 16 x 16 / 32 x 32
And the Gpu around 256 x 256 / 512 x 512
Thanks for the information! 
I did a check after reading your post. It turns out that the CPU functions the best for speed with my setting at 16x16 with a time of 2’ 37.25" with 32x32 coming in just behind at 2" 38.92".
The GPU did not fair any better in rendering sizes than what I had set at my best time which was 480x270 with a time of 1’ 19.28". The closest was 512x512 coming in at 1’ 19.3".
I am not sure which is best to use as far as tile sizes. Is square tiles better than rectangular tiles which fit the dimensions of the output file (being 1920x1080).
Thank you again for responding to my post.
In most of the videos i saw about rendering, i saw them use square tiles 
But if you want to know more details about it, maybe Michael knows more about it 