First problem: You have 400 pixels width to go on, and need to fit some text within that constraint as large as possible (thus, the text shall use that amount of space).

Throw in a new constraint: If the text is just "A", then it shall not zoom this above 100 pixels (or some specific font size).

Then, a final situation: Linebreaks. Fit some text in the largest possible way within e.g. 400 x 150 pixels.

An obvious way is to simply start with point 1, and then increase until you can't fit it anymore. This would work for all three problems, but would be very crude. The fitting of a single line within bounds could be done by writing it with some fixed point size, check the resulting pixel bounds of the text, and then simply scale it with a transform (the text scales properly too then, check out TransformUI).

Any ideas of other ways to attack this would be greatly appreciated!