What is the recommended size for a photo/avatar, e.g., online forums? I've seen a range of suggestions, e.g., 40 x 40, 80 x 80, but not sure if there's a standard recommended size. Thanks.
There's no standard across the web. Go big and scale down rather than scaling up from a small image if at all possible.
I like to have 50x50 pixels
That's a good size, but on some websites, it would be scaled down and end up looking more like an icon than an avatar. It is really best to see the website's guidelines for uploading an avatar. I think Gravatar.com is probably one of the best resources for crafting a personal avatar that is compliant at many websites.