Filckr and SmugMug

I will leave the scientific explanation to someone else but when I look at that image, which is 800 pixels wide, I get a 200 mm wide image on my monitor. (20 ins size 1920 x 1080) with the image size option selected to original or auto

My original P base uploads were at 800 pixels on the longest edge which seemed OK at the time. P base was a bit more restrictive on image size then and my upload speed was slower; although it is still less than 1MB.

Now, I upload images which are mostly around 1200 pixels. That seems to be a reasonable size considering that many people will be viewing on small monitors with phones or basic laptops etc.