I have only one 1070 video card and I am using the GPU compute.
When I set the tile size to 64x64 the render time is 17.56 second for a single frame.
If I change it to 300x400 and render the very same frame my render time is increased to 32.62 seconds.
I did both tests without checking the denoising check box.
Hi Elisha, as far as I know tiles should preferably be square.
They should be large for GPU and small for CPU.
Furthermore they should ideally fit in the image resolution, so for a 1920 by 1080 resolution, which is 16*120 by 9*120, you could use tiles of size 120*120 (for GPU rendering!), or 24*24 (for CPU rendering)
The ideal size does depend on your GPU, but the above rules should be followed for the best results.
You could also just use the Render: Auto Tile Size Addon, that comes with Blender. (I am not sure if it also considers what GPU you have...)