Not sure why that's near impossible. Admittedly I don't have said Sentinel myself, but I imagine a fair few others do...
While I can't do this, I can tell you for sure. The combiners are big, but not that big. I think they come up to his lower torso, but someone will post pics...
I don't have the energon combiners, but i do have a sentinel... anything else you wanted as a size ref?
Not the best pic, but the lighting in my room is horrible... and I didn't know what else to take pictures of.