Alright. I asked because it could be an XY problem and I cannot know what you need this Y-size for nor how frequently.
GetBoundingBox() is definitely the quickest method, but if having the mentioned world axis-aligned primary part is not an option, then you can use one of the bounding box algorithms.
I recommend taking a look at XAXA’s function, derived from the engine source.
It comes with a caveat. Engine calculations are always going to be much faster. AABB operation complexity is linear. Even if completely optimised and compiled as --!native, this function will be around two orders of magnitude slower at best. With each additional part the execution time grows, while it hardly changes for GetBoundingBox(), even in large models.
Edit. Just below the XAXA’s post Pyseph added a faster version.