While working in query pane of your favorite SQL Server Management Studio (SSMS), you may be required to change font size in SSMS query pane. For this click on Options in Tools menu of SSMS.
Options frame would be opened. Click on Fonts and Colors under environment as shown in snap. On right side you have font size drop down along with several other fonts and colors related settings.
Adjust the font size and Click Ok. changed font size would be immediately implemented even in already opened query panes.