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 this bbcode to adjust the image sizes without needing to edit them manually:
[img height=100]https://bitcoin.org/img/icons/opengraph.png[/img]
https://bitcoin.org/img/icons/opengraph.png[img height=50]https://bitcoin.org/img/icons/opengraph.png[/img]
https://bitcoin.org/img/icons/opengraph.pngJust change the height value.
what about those direct link on logo? Like if i want to post a discord then when readers clink that discord how theyll directly catch that discord? How can i set that?
