Resize just one staff in a score

Hello,

Here's my problem: I created a score in Musescore and resized the staves to make the notation more easily readable for the children who will be playing the music. I had more or less completed the score when I realized I needed to add another instrument. Now that I have added this instrument/staff to the score, I have been unable to make it the same size as the other staves that I created when I began the project. I have tried the usual methods of changing staff sizes such as the Page Layout - Scaling - Space option and the Edit General Style - Sizes option. While these options do change the size of the staves overall, I am still left with the problem of one staff being slightly smaller than the rest. Is there anything I can do? Any assistance would be much appreciated. Thanks.