[plt-scheme] message%'s

I've created a guibuilder frame, and now i'd like to abstract over it.
 I've got message%'s which via GUIbuilder says "First frame" on
creation.  Now i'd like to

        (send message-thingy set-label "Second frame")

but "Second frame" is longer then "First frame" so it gets cut off,
and looks more like "Second fram".  Now I can:

(send message-thingy min-width 200)

Now the whole thing can be seen, but it's too big, and the spacing is
all out of wack for the frame.  So is there a way to calculate how
wide text of a given string would be, so I can set min-width to that?

Corey