How can I change the font size of only a specific part of a Microsoft Word document?

There are a few different ways to do this. Here is the task referenced, just spelled out in detail. Search and replace for specific formatting traits within the document. From within the document, press CTRL+H to bring up the Find and Replace dialog. Click the More button at the bottom of the dialog to reveal the Search Options. Point your cursor inside the Find what field, then click Format > Font to bring up the Find Font dialog. For this specific problem, within the Size field, type 7, then click OK. The text Font: 7 pt appears under the Find what field, indicating that it will search for only text that has a font size of 7. Repeat steps 2 through 4 for the Replace with field, and make sure to indicate size 12 font in the Find Font dialog. The text Font: 12 pt appears under the Replace with field, indicating that it will replace size 7 font with size 12 font. Point your cursor anywhere inside your document, leaving the Find and Replace dialog open. Press CTRL+Home to go to the beginning of your document. With both the Find what and Replace with fields blank, click Find Next to search for the first instance of size 7 font. Click Replace to replace this instance of size 7 font with size 12, then click Find Next to go to each instance of size 7 font, or you can click Replace All to replace every instance of size 7 font with size 12.