User blog comment:Matt Hadick/Ultimate Battle Final Round/@comment-24.16.149.136-20130709220120/@comment-4738647-20130710010924

Because there are no official stats, all we have is his official altmode an FL86 Frieghtliner and common sense on how he Transforms, Optimus is 20m minimum.

A problem is Transformers can change size, the technology employed is also how Optimus hides his trailer. It is useful for disguisng yourself as a small tape recorder but it isn't that useful to shrink in robot mode so Optimus cannot become smaller only bigger.

What do you mean he doesn't have the matrix naturally? Why is that even relevant, he is the leader for most of G1 so he has it in him for most of G1.