Judging from your screen shot, the issue is not the score system size, but rather the page size.

Document menu > Page Format > Score…

Finale uses the settings in Page Format for Score when creating new pages.

It looks like you have changed the the Page Size on Page 1, but you have not changed the settings in Page Format for Score.

When Finale created the following pages (2, 3, 4, 5, …), it used the (old) settings in Page Format for Score.

Fix:

Page Layout Tool.

Go to Page 1 (= the page with the Page Size you need).

Page Layout menu > Update Page Format for Score (select this)

NB: {Update Page Format for Score} is a program specific setting, not a document specific setting.

Before you leave this document (and begin working on other documents), you should probably de-select {Update Page Format for Score}.

Page Layout menu > Page Size…

Change the Page 1’s Page Size to some other Page Size.

Then, change the Page Size back (not via Undo!).

Now, when you go to {Update Page Format for Score}, you will see that the settings have been updated, to exactly the settings on Page 1.

When you are done, you can de-select {Update Page Format for Score}.

You will not see any change on the following pages.

This is by design, and what users have requested.

To update the following pages with the new settings:

Page Layout menu > Redefine Pages…