This issue is only affecting one of my WordPress.com hosted blogs.
I made the window in the visual editor longer while editing a post, and now it is "stuck" at an excessively long setting. Grabbing the resize control to restore it to a shorter length appears to work, but the length returns for new posts, or if editing draft or published posts.
This problem only affects me, and only on this blog; other contributors have no issues with the editor.
The blog I need help with is arabliss.wordpress.com.