Did you get stuck with this tiny window when launching Arena?
If so, the fix is quite simple. Follow the steps below.
Close Arena (or that tiny window)
Press the Windows key + R
Type in regedit then click OK
Navigate to: (screenshot below)
HKEY_CURRENT_USER/SOFTWARE/Wizards Of The Coast/MTGA/
Delete the following items (Right-Click each one individually, select Delete)
Start Arena back up again, it should launch correctly this time.