You cannot expand the display width (field width) of Related Records.
Display width is automatically adjusted by your Web browser based on the value of the field.

For your reference, you can adjust the display width of the Related Records field by adding spaces to the field names in the app specified as "Datasource App" in the Related Record setting.
The steps are as follows.

1. Open the app specified as "Datasource App."

2. Click the App Settings icon (the gear-shaped icon at the upper right of the app's "View" screen), then select the Form tab.

3. Place the cursor over the gear-shaped Settings icon at the upper right of the field for which you want to adjust the display width, then click Settings.

4. Add full-width spaces (empty spaces) at the end of the field name to lengthen the field name and then click Save.
   Make sure that you enter a period at the end. Without a period, added spaces will be cut off when the field name is displayed.

5. Click Save Form on the upper left side of the screen.

6. Click Update App at the upper right of the screen.

7. On the confirmation dialog, click Update App.

The display width of the Related Records field is expanded to include the added space.