I was wondering if the Iceform shout, with all three words, could freeze a dragon. I don't want to get disappointed by it if I am going to get it.
According to the Skyrim Wiki:
This effect will not freeze Dragons nor Mammoths, only cause some frost damage.
The Ice Form shout will not freeze dragons or mammoths, even with all three words unlocked. Using additional words in the shout will increase the duration of the effect on the target, which directly affects how much damage they will take from being encased in ice.
At level one, it will freeze an enemy for about 10 seconds. Each additional word appears to increase the freeze time by 10 seconds. The cooldown starts at 60 seconds and increases by 30 seconds for each additional word, thus there is always a gap between the enemy unfreezing and you being able to use the shout on them again. Multiple enemies may be hit by the shout.
Enemies receive damage at a rate of about 1 point per second. I believe this counts as frost damage, so apply appropriate resistances/weaknesses to this value. Striking the enemy before they unfreeze breaks them out early.
If you're looking for shouts that are fun to use against dragons, I would recommend Storm Call, Slow Time, Elemental Fury, Dragonrend or Call Dragon.