Per the Adobe documentation for Text
Sizing a Text control
Flex sizes the
Text control as follows:
If you specify a pixel value for both
the height and width properties, any
text that exceeds the size of the
control is clipped at the border.
If you specify an explicit pixel
width, but no height, Flex wraps the
text to fit the width and calculates
the height to fit the required number
of lines.
If you specify a percentage-based
width and no height, Flex does not
wrap the text, and the height equals
the number of lines as determined by
the number of Return characters.
If you specify only a height and no
width, the height value does not
affect the width calculation, and Flex
sizes the control to fit the width of
the maximum line.
As a general rule, if you have long
text, you should specify a pixel-based
width property. If the text might
change and you want to ensure that the
Text control always takes up the same
space in your application, set
explicit height and width properties
that fit the largest expected text.
So the trick I've used to deal with this is to have the Text get its width via a binding expression from whatever container limits it's width, typically the immediate parent.
e.g.
<mx:Canvas id="box" width="100%" backgroundColor="Red">
<mx:Text width="{box.width}" text="{someReallyLongString}" />
</mx:Canvas>