Oh sweet lord, excuse me!
The PST on it's own is approximately 15 inches from back of black box to front of the objective in single stack configuration (38cm).
The PST from the bottom of the black box to the top of the eyepiece holder without an eyepiece is approximately 5.5 inches (14cm).
The double stack module adds another approximate 4 inches (10cm) to the front of the PST's length when threaded on.
So if you want to look at it like that, it's approximately 20 inches long by 5.5 inches tall and it's less than 3 inches wide.
Add foaming protection around those dimensions and you're set.
You can actually make it all fit in the original PST box, including the double stack module. Just not threaded onto the PST.