Blackout is close in size to an upright Starscream. Starscream comes in around 31 feet and Blackout around 33 feet. I've seen fansites showing Grindor the palllete swapped Blackout coming in around 50 feet. If you watch the forest battle in ROTF in a few shots he lumbers around and is clearly larger than Megs and Starscream. Also Optimus (28 feet) looks smaller when he's sitting on top of him ripping his face in two and drops off.
For all Bay's talk about size displacement Starscream and Blackout's alt-forms are much larger than their robot forms 63 and 91 feet respectively. I was surprised to know Soundwave is the same size as Ironhide. He always looked more like Bumblebee's size in DOTM. Megatron in 1 and 2 doesn't suffer from a size displacement due to his forms being Cybertronian.
Here's a 53 second video of Grindor in ROTF for size comparison and a size chart for Blackout from the first movie.
https://www.youtube.com/watch?v=Dmy7qzMhq18