FAQ:
4.1.6 When does a mage have to specify his spell's target?
When he rolls the dice. In fact, the only thing you have to declare when you first start to concentrate is what spell you're concentrating on. The exceptions are spells like Divination and Seeker, which are worded so as to imply that one must start out meditating on a specific target object or question.
In general, the following things can be left up until the instant the dice are rolled:
Area affected (Area spells)
Content (spells like illusions, Voice or Delayed Message)
Energy source (i.e., ST, HT or Powerstones)
Level of effect (spells like Shield or Teleport Shield)
Destination (Gate magic and Teleport spells)
Dice of damage (spells like Deathtouch or Flame Jet, but not Missile spells, which have their own special rules)
Hit location (spells like Paralyze Limb and Wither Limb)
Subject (most spells)