labeling - Rule based labels using QGIS?

I couldn't find an option of "rule-based"-labeling in QGIS 2.10. For example that big cities are also labeled bigger than smaller cities. Am i missing something or doesn't it exist?

Answer

You can use expressions to define a font size

For example:

CASE 
WHEN "size" > 10 THEN 10 
WHEN "size" > 20 THEN 20
ELSE 30

END