I "guesstimated" 1/2 inch clearance front and back and sides using an "L" square on the ground and looking from above (i don't have a caliper that large, and you want to consider the pull starter and padding in your guesstimation), and also how high I wanted the blowers to have to bounce (exactly straight up) before coming out.
I came up with (43"L) x (14"D) x (14"H)
one of the front supports keeping the blower from falling into the trailer is an inch lower because I simply had pieces that were already cut.
I should be able to start these while still in the box- saving me time and bending over at each stop.
Liberal application of Rustolium Gloss Black (which, I think, everyone should have in thier garage in bulk