iphone x camera files sizes
If you’re not a heavy user, 64GB is more than enough for what you need. Here is the size of videos in different resolution

1 minute 1080p at 30fps will be 60MB
1 minute 1080p at 60fps will be 90MB
1 minute 4K at 30fps will be 170MB
1 minute 4K at 60fps will be 400MB
(Approximately)
More resolution can be seen in Settings -> Camera -> Record Video

One high quality photo from my phone is a little more than 20MB.

Unless you’re filming at 4K60 all the time (for your job or something), and you don’t use big apps, 64GB is good enough for you. Otherwise go for the 256.