Hi Jan I just did a quick check real quick to see if we could check on file size.
Here is what I found in the help section:
If your image won't upload, try these things:
Check your file type. Twitter supports JPEG, GIF, and PNG file formats. We've had difficulties with animated GIFs, so we do not support them. All existing animated GIF images will be left as they are, but no new ones can be uploaded.