If you would like iTunes to automatically download movies and TV shows purchased from the iTunes Store in full size HD or high quality SD, you will need to make some minor adjustments in the program’s settings. Here’s how configure iTunes default download preferences.
Download HD Movies from iTunes
Open iTunes and sign in with your Apple ID.
Click on the Edit menu, followed by Preferences.
Next, go to the Store > Automatic Downloads. Tick the Download full-size HD videos and Download high-quality SD videos checkboxes: