This appears to be the most accurate scale guide I’ve ever found.
The top image of Starscream standing next to MECH agents is the basis for these measurements. Assuming that Silas is 180cm (about 6ft) it extrapolates that Starscream is between 720cm (about 24ft) and 760cm (about 25ft.) For the sake of simplicity they seem to have gone with 750cm.
How's it hummin'? 