The Enlarge Spell feat allows you to increase your at-will/encounter burst and blast attacks by 1 if you take a -2 damage penalty per die.
Before using a wizard at-will or encounter attack power, you can choose to take a -2 penalty to each die of damage rolled with the power to increase the size of its blast or its burst by 1.
You can't use this feat on a power that doesn't roll dice for damage.
What happens if you get a critical hit though? Would the penalty be -2 times the dice from the original attack? Or -2 times the amount of dice actually rolled? Would this include the Critical dice added with most magical items? What about the dice from an Empowered Crit implement?
My assumption from the exact wording of the power is that it is the exact amount of dice rolled with the power, including the bonus critical ones, but not counting the ones that are automatically maximized, however, I though I would double-check.