So, these are all "back of the envelope" estimates... The actual measurements will vary but it will give you an idea. You should maybe send an email to Park or stop by your LBS if they carry these stands confirm the measurements and check my math.
Assumption 1- Leg Length
From the park tool site link it says that the whole stand folds to 41". If you watch the video you see that when folded the head and mount stick up above the legs. It's likely more than 5 inches, but I estimated that if you account for that excess the legs are about 36" long.
Assumption 2 - Right Angles
My next assumption is that the legs open at a 90-degree angle. Then we can do some high school math.
Do the Math!
If both legs are 36" and the angle is 90-degrees, then then length of "b" in you diagram can be figured with the Pythagorean Theorem, 36^2 + 36^2 = b^2. My math tells me that "b" equals roughly 51".
If you put the pointy end against one side of the tub and legs against the other side, you then create the smaller triangles formed by the legs, the vertex "a" and 1/2 of "b". Again, using the same formula you have 36^2 + a^2 = 25.5^2. That equals roughly 26".
So, based on those assumptions (right angles and 36" legs), if your tub is wider at the interior base than 26" (66 cm) then the stand should fit.
Edit: If the length of the legs is about 25" as indicated in a comment that has since been deleted, then the length of "a" in your diagram is about 17.5" (44.5 cm).