I took the seat post out of a Serial 1 today. To my dismay, it was not the 30.9 that my current e-bikes use, so I couldn't plug in my suspension post and seat.
But I did have a tape measure. It looked to be about 1 and 1/16th inch, which is about 27mm. According to Wikipedia, "The most common size is 27.2 mm (1.07 in) for most bikes, especially for the higher-quality models".
You can also email Serial 1 support (support@serial1.com). I have found them to be extremely helpful.