Nopz. In Windows 8: Open Regedit > (Keyboard: flag+R) > Regedit

1. HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\FontSubstitutes\MSShellDlg and replace the font that you want!

2. HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\FontSubstitutes\MSShellDlg 2 and replace the font that you want!

and the The font size?

well...go to Control Panel > Video > Advanced Appearance > and be happy!

Enjoy!

In Windows 8

1. Open regedit (press Win+R and type regedit)

2. Edit the following value and replace the font that you want:

   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\FontSubstitutes\MSShellDlg
   

3. Edit the following value and replace the font that you want:

   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\FontSubstitutes\MSShellDlg 2
   

As for font size, go to Control Panel → Video → Advanced Appearance, and change it there.