I saw that you can now put a cover photo for your steemit profile (In settings - public profile settings - cover image URL) and went ahead and quickly added one. The result was messy hence you have no crop or position options. Than I debugged for a second and found the result while inspecting the element.
In the debugger I inspected the cover photo and in the menu selected layout to see the exact size for the photo.
And that's how I got my result of 1354 x 155 pixels. Hope this helps and soon you get a nice cover shot too!