I was going to say, if you can't figure out iMovie then you shouldn't be even looking in the windows of Final Cut. Final Cut is very capable and not so fresh off the boat user friendly.
You can try to download handbrake (free software) and run your raw GoPro files through there first. There are multiple formats within handbrake to save your re-run but some times GoPro footage becomes corrupt and needs a handbrake run to clean things up. I don't think file size has anything to do with it but I have noticed that larger files have been getting broken up into smaller segments quite a bit lately.
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