Automatically scale video on import to sequence resolution
You can achieve this by the using Default Media Scaling preference, which takes effect when you subsequently import media and/or add to the timeline.

Edit > Preferences > Media > Default Media Scaling and choose "None", "Scale to Frame size" or "Set to Frame size"

Scale to Frame size: Will resize the media clip virtually at import - such that when added to the timeline at 100% it fits the frame.

Set to Frame size: Will set the media scaling dynamically using the effects control, such that at 50% (in your example) it fits the frame. Importantly - you can then rescale the media up on the timeline without loosing the original quality.

You can also use Clip > Video Options > Scale/Set ... (with clips selected in the Media Library or Timeline) to rescale clips in the manner stated above.