Here's what we know:
* In the comics: He's often shown as being smaller than most other Transformers, but not significantly so.
* In the Animated Series: He appears to be a bit smaller than Optimus Prime and other Autobots.
* In the live-action movies: He's not featured in any of the films, so we don't have a visual reference for his size.
Ultimately, his size is mostly left up to interpretation, and it's not a crucial detail to his character.