There is no such thing as a standard "best size". It completely depends on what the image you're posting, and for what purpose. Mostly though, as long as we could see what you're trying to show us in an image, then it should be good enough. Just don't post those unnecessarily huge ones that fills out monitor screens.
You can use these to adjust the image sizes:
[img height=500]*image url here*[/img]
Just change the height value.