simply open style.ini, goto standard button section change font size
Code:
[StandardButton]
Width=220
Height=60
Font=Calibri
FontUC=1
FontColor=$FFFFFF
FontColorOnMouse=$dddddd
FontColorMouseDown=$ffffff
FontColorDisabled=$000000
FontSize=12
FontBold=1
OneLiner=1
Alignment=center
LabelX=0
save the file & then copy it into setup folder ( conversion folder ) overwrite file
then compile BOTH installerscript.iss & langscript.iss