After importing just drag one of the clips to your timeline rather than worrying about correct settings.
If you get a screen asking what to do just cancel/close it.
After any editing you export to set the final size for uploading or saving.
There are some presets will help but go through the list of options and make sure some basic settings seem OK e.g. frame rate.
Try a few very short clips which render and upload quickly, and see how it turns out.