I look forward to that answer and also for photos uploaded (added) to discussions.

What is the best size?

Depends what you mean by size, but if you take a look at "upload a new profile" it clearly states 1MB.

Then it's up to you whether to go for size or quality.

As far as scrapbook is concerned - I'm not sure there is a limit on the one on the front page of your scrapbook - but you are limited in any further additions to that page.

Which is why some people have chosen to create a lot of scrapbooks with only one picture in them.

That's my experience anyway.