I've attempted to reverse-engineer the dimensions from screenshots, but this method has proved unreliable. I've found no trace of any record of the Combine synth gunship's dimensions online, and have no way of checking for myself in-game.
I am particularly interested in the nose-to-tail length of the gunship.
I opened the Hammer editor and added a Gunship to get the size of its bounding box, which should give us an approximation of its nose-to-tail length:
These are in "Hammer Units". According to the Valve Developer Community, there are 16 Hammer Units in a foot for architecture, but 12 per foot for human character models. This gives the length as something less than 45 feet.
To be a little more specific, I rotated it and got as close to the length you wanted (I didn't include the barrel on the front, but I did include the tail):
530 units gives us a length between 33 feet (for 16 units per foot) and 44 feet (for 12 units per foot).
