I want to shorten the width/length of the measure with a whole note in it.
This isn't really controllable on a per-measure basis, but you can change it globally (on an entire file) using File | Page Setup | Options tab. Uncheck the box "Increase note spacing for larger note durations."
Yeah, but when you change it globally, every note takes up the same amount of space, then you have to insert a whole bunch of "blanks" to make it look right.
True, of course. But not knowing exactly what the OP had in mind, the first answer was my best-guess.
A facility that's often useful to spread things back out is to force system breaks. Select the barline (has to be the top-most staff) and edit the barline (select it and ALT-ENTER or CNTL-E), check the "Force system break" checkbox.
Even with "Force system break", every note takes up the same amount of space.