By "part" do you mean system? It's possible that the staff was resized accidently. Try using the resize tool to correct it.

To piggyback on Mike's comment, I like to take the Resize Tool and right-click the previous system (which looks correct), and apply its values to everything that comes to ensure everything's as uniform as possible!

Chad's solution of the Resize Tool is correct.

Unfortunately, Finale remembers the last system size change you made, and makes EVERY new system that size, whether it's the same part or not. For example, if I resize my piano part to 80% instead of 85%, then the NEXT part I start to format remembers that setting. So if I stretch out my bass part to 11 systems from 10, the 11th system is sized at 80% instead of 85% as expected.

This apparently is the way Finale was designed, but for the life of me I can't understand why. It seems much more intuitive to me that each part should preserve the SAME staff/system size for new staves/systems as were previously in the same part. 

Agreed. I have never understood why new pages don't automatically take their setting from the preceding pages. Just another thing that doesn't make sense, and will probably never be changed.

Hey guys!

I agree that it's puzzling why this wouldn't always happen automatically. In nearly every case, I want my new pages to match my old pages, too! Here are couple of suggestions I've found to help minimize these effects:

1. When using the Resize Tool to resize systems and/or pages, make sure the checkbox labeled Update Page Format in the Resize dialog box is checked. This updates the underlying values that determine what new pages look like.

2. Similarly, when using the Page Layout Tool, make sure Page Layout > Update Page Format for Score is checked when making any edits.

There was some reason why I would not check "Update Page Format" that was very good, but I can't remember it. It was discussed in an exchange with tech support a couple of years ago, which I would reference here except that MakeMusic discarded all my old interactions with tech support. Can anyone think of what a side-effect to checking that box might be?

Would checking the option perpetuate the new settings to other subsequently opened files where they were not wanted? That would be my concern.

Ah ha! I found the reason why I won't check "Update Page Format"!

It's because Finale then will read the default page format values, EVEN IN THE PART WHERE YOU RESIZED!

So let's say I resize my piano part to 80% with Update Page Format checked. I can go back to edit my other parts and add systems with impunity, but when I go back to my piano part, any NEW systems I add at that point will be at 85% (my default staff reduction) INSTEAD of the 80% I want!

This is the way it should work: if you reduce a part, then any new systems added later should inherit the reduction that you already set IN THAT PART ONLY, not in EVERY part. ANY part should continue to add new systems with the SAME staff reduction as the previous staff, regardless of what you have just set in some other part.

Good point, Christopher!

The way I like to think of it is that the Resize tool works well a la carte to use on specific systems, and "Updat[ing] page format" will update the values in the Page Format for Parts (or Score) dialog box depending on where you are which, as Christopher mentioned, can be a bit counter-intuitive. 

However, it is sometimes good to keep in mind (mostly when using the Page Layout tool or the Resize tool to change formatting in all parts) that having that check box for "Update page format" could save you some headaches - just be sure to answer the question of "Is this edit for a particular part/part of that part, or is this supposed to be a global edit to affect all parts or the score including for all newly created pages/systems?"

All of that being said, I see where you are coming from concerning the inheritance of particular edits needing to affect future systems of JUST that part in some cases. Finale does not currently have an option for that, and perhaps it should! I'd be happy to go ahead and take note of that feedback.

Am I correct that Update Page Format is an all-or-nothing option that cannot be limited by choosing specific pages in advance?

That is correct J Adrian. It updates either the Page Format for Score (Document > Page Format > Score) or Page Format for Parts (Document > Page Format > Parts) values which dictate the values for all newly created systems, pages, etc.

You can still use the Page Layout tool (in this case Page Layout > Resize Staff Systems (System Reduction)) to apply changes to specific pages/systems in selected parts/score, but updating the default values in the page format dialogs (using "Update page format" or manually) is all-encompassing for the future.