If you are creating a movie in Screenflow then adding titles and effects in iMovie
Use Free Ruler to open a Firefox browser window at exactly 1024 (wide) by 850 (tall).
Start the recording (turn off video camera and mic capturing if not using).
Stop the recording using the key combination CMD + SHIFT + 2
Edit in Screenflow, export as 100% and Losless (rather than Web - High or Highest).
Import into iMovie. Create a new project in standard size (not wide).
Crop your footage to fill the screen.
Export as Quicktime as 1100 x 845.
Rename the file extension as .mp4
Upload to Sitebuilder.