On the picture size, 800x600 has been recommended for a while, but most people have much bigger monitors in the last 10 years, and also faster internet service.
I have standardized on 800x600 on my site to make it load faster, and work on smaller screens, were I to start over today, I would go 1024x768.
I don’t see people complaining a lot about larger pictures, and when they do, they are definitely in the minority, so it really depends on your use of the picture… everything I shoot starts in 6000x4000, which usually allows me to crop the picture to just the points of interest.
Greg