No, unfortunately not. The width of the value display box is currently
hard coded with a fixed size of 35 pixels.
This is a known issue that should be improved in FLTK 1.4 and there is
STR #3222 that provides a patch for 1.3.4:
http://www.fltk.org/str.php?L3222
http://www.fltk.org/strfiles/3222/Fl_Value_Slider.patch
For now you can try if this patch can help you (it makes the value
display width user settable). It applies cleanly to FLTK 1.3.4, but I
didn't really test its behavior. It looks correct at a first glance.
Anyway: if you use it please be aware that the given API may change in
FLTK 1.4 if it is included in the library.
Advanced mode: If you don't want to patch your FLTK library you can
derive your own widget from Fl_Value_Slider and override the draw() and
maybe also the handle() method...