Learn English – the meaning of ‘a window from seven to four’ in this sentence

This is the passive participle of the verb sink, which literally means to make something sink in water, usually through the use of a firearm in combat. Figuratively it means to go down, lose, get into trouble, etc. as a consequence of something.

This is a common way of predicting someone's demise, death, defeat, etc. by already pronouncing them as such in the present. For example, a child breaks something while his parents are out and says: I'm dead! Obviously, he's not (figuratively) dead yet, however he is announcing what he considers the unavoidable consequence of his actions by pronouncing himself dead in the present.

Similar expressions used in this way are:

I'm dead

I'm sunk

I'm a goner [>gone]

I'm finished

I'm done for

I'm history

I'm screwed

I'm fucked

It is short for "That is you done". It signifies that Madam Malkin has finished putting pins into robe that Harry is wearing. It is a common (UK) English phrasing used to mean that someone has finished the work, or a stage of the work, that they are doing on behalf of another person. Here the pins have been placed to show the wanted length and next someone can sew the robe to that length.