You're font size/zoom is set larger than the default:
1. Click View
2. Then "Text Size" (or "Zoom")
3. Then "Default" (or "Reset").
Also, you're OS's DPI maybe set larger than the default. This causes everything to appear larger across all windows, and breaks MiceChat's look, if you want to reset it:
1. Click on Start then Control Panel.
2. Click on Appearance and Themes and then click on Display.
3. A box will pop up. Click on the Settings tab. 4. Click on Advanced.
Change the DPI setting to Normal size (96 DPI). Click OK and your computer will ask you to reboot your computer. Click Yes.