SSRS 2005 : How to format a textbox using a conditional formatting?

• In the layout tab, select the textbox to modify
  
• click on the arrow of the FontSize property to deploy the combobox
  
• click on Expression to open the Edit Expression window
  
• In this window, you can write a iif clause for instance as

IIF(Fields!KPI.Value ="The world","10pt","12pt")

• You can do the same sort of formatting with the font, the color, the value, etc.