You can't really change the font size separately in the "Search and Replace" window. Also, there's no need to change the whole Visual Studio window size. Instead, you need to adjust the font for the environment and text editor separately. Here are my recommendations:

• Use Segoe UI 10 or 11 as the environment font (Tools > Options > Fonts and Colors > Environment). Segoe UI font 9 or 10 will be installed in the "Search and Replace" window.
• Choose Cascadia Code 14 as the text editor font (Tools > Options > Fonts and Colors > Text Editor).