Where do the motors mount on the top beam? At both ends as far as possible?
The 66" 2x4s that make up the frames height, would I be able to cut down to
about 60"? If so, I would have a place to store the maslow in my garage.
I started off with the ‘that looks about right’ criteria, but the more I dig
into the option of shortening these, the more it looks like this is really the
you want the motors to be ~18" above the top of the work area (you may be able
to cheat this down to ~12", but that will mean the motors are really working
hard to move the sled to the top center), and you need ~9in below the edge of
the bottom beam to give the sled room to move down far enough for the bit to cut
to the bottom of the work area.
so 5" for wheels , 1.5" bottom 2x4, 3.5" for the 2x4 that supports the
workpiece = 10", just about the minimum for the lower spacing.
you then have 48" for the workpiece and 18" to the top of the top beam, for a
total height of 76" (plus a couple inches for the motor, so call it 6’6" total)
out of this 76" you have -5" for the wheels, -1.5" *3 for the bottom 2x4, top
2x4, and main beam = 76 -6 - 4.5=65.5" for the vertical supports.
so you could cut the 66" verticals down by ~1.5" but anything more than that and
you are going to run into problems, either at the bottom of the work area, or
you could tweak the sled a bit to give you an extra inch or two of space at the
bottom, and you can probably get away with cheating a little at the top. So if
you have space for one with 60" verticals, but not 66" verticals, build what you
have space for. Just be aware that you may run into grief at the top center, or
with the sled hitting the ground at the bottom, depending on how you tweak
 note, taller wheels work well, and if you use taller wheels, keep the
distance between the top of the bottom workpiece support to the ground the same,
and move it down on the wood and shorten the vertical pieces.