Legend:Autosize property must be set to False to play with the Legend:Size.Width property.
Now if what you really want is to control the total height when the panel resizes... (And my magic 8-Ball says "Sure, That's exactly what the OP wants!")
Set Legend:scrollbar visible =TRUE (This will allow the user to see any plot in the legend regardless of resizing)
Add an event case for "Panel Resize"
In the panel resize event case; read the new panel height, do a bit of math and set Legend:Number of Rows to the floor of panel height divided by height of one plot name in the legend
"Should be" isn't "Is" -Jay