In LabVIEW, drop a property node from the Application Control palette. Right click on it and choose Link To and navigate to the graph you want to change. Now click on the property text. This will activate a huge menu. Down at the bottom, choose Legend->Size->Width. Right-click the new property text and choose Change To Write. Set this value to a width in pixels. Note that the optimum value will change depending on your display font (whose default changes between XP and Vista).