I'm sorry, but we do not have a set size. In general, it should probably be a rectangle (wider than it is tall).

You have developed a web page, and it only looks good at certain sizes-ranges. So, you should know these and be able to give a guestimate of what you will be wanting.

@bitcoiner and I do not work for Steemit, and we did not develop the site. We are just two people from the community that are trying to help contribute and make it better.

There are quite a few different sizes that should work. We plan to try out whatever people send us.