final cut pro x screen is small

Tom is absolutely right, as always :-)

You can see it more clearly from your first screenshot. Notice how the navigation rectangle is tall instead of wide:

Select your project in the browser, open the Inspector.

You will see the project dimensions.

You can click the Modify button and change the dimensions so it is horizontal instead of vertical.

The project is vertical.

Select the project, click Modify and see the results.

You are not going to mess anything up, but if you want, you can first make a Duplicate of the project. That way, if necessary, you can use the Duplicate.

What is t the frame resolution of the original media? The second screenshot makes it look as if you have landscape, horizontal video in a portrait, vertical project. What are the project properties?

In the first screenshot, your viewer is scaled DOWN a lot.

In the second, it is scaled UP a lot. Since it is scaled, it doesn't fit in the viewer, and that is why that red navigation panel is there.

NOTE: this only affects how the video is displayed inside the viewer, it does not affect how it will export.

You can choose the scale at which to present the viewer:

"Fit" will adjust depending on the available space:

And you can change as needed:

Again, this changes ONLY how things are displayed inside FCP.