How do I check the size (in MB or GB) of the video clip in the timeline quickly?

The final 'size' depends on your ecport setting - one project could be 7.5MB or 22.99GB in size -...

So, when you goto Share/selecting your export format, you should see in the following dialogue the estiimated size ...

My 2 cents on what is where:

What is in the Timeline are not actual media files but instructions about where to find media files and do something with them.

That's what a non-destructive editor like FCP X does.

Clips in the Browser and Timeline are representations of actual media files that are stored on a drive.

Does this makes sense?

To clarify:

Right click a clip in the Timeline and select Reveal in Browser, a Browser Clip will highlight.

Now right click that Browser clip and select Reveal in Finder, a clip or media file will highlight, this is the actual file that FCP X is using in the Timeline.

This REAL media file will have a size in MB or Gig and does not change.

The files or pointers in FCP X don't have a "size" in the same way.

If you assign in/out points to a Browser clip and edit it to the Timeline you are saying, "find this particular clip on a particular hard drive, start playing here and finish there."

You can change the in/out points all you like and the original real file is unchanged or Not Destroyed. Hence non-destructive.

Al