Is there a 'best/optimal' image size for the 'full screen slider' images?

Hi,

I use the full screen slider but it looks like that the images is a bit ‘blurry’ or not so sharp when I upload the,
My original images are min. w6000 x h4000 px. so they are quite big.I open them i Photoshop: downsize them to reduce filesize and I export them as web optimized images. Here they look really good and really sharp.

But then when I upload them and look in the frontend they look not that sharp and also it looks like that it has been zoomed in.
I beleive it will automatically fit the size of the screen, but it looks a little stretched.

So, my question is simply if there is a optimal image size that I can set in Photoshop: e.g. 1600 x 1000 px. or another format.

Thanks in advance