You need a "seat post" which has to be exactly the same diameter as the old one.
The specs page only says "Seat post: STYX aluminum" which is not enough information.
Your best option would be to close the clamp to its normal position, and measure the internal diameter of the frame using calipers. A ruler won't be accurate enough, because while common seatposts could be in these sizes:
- 25.4 mm
- 26.2 mm
- 26.8 mm
- 27.2 mm
- 28.6 mm
- 31.2 mm etc
It is not impossible for the seatpost to be some weird in-between size.
The overall length of the seatpost is less important, only that it is long-enough.
You'll also need a seatpost with an integrated clamp at the top, or a separate clamp. Very likely whatever you buy will have a clamp included.
Separately - someone has stolen your bike seat once already. Expect that it will happen again. Some relevant links to protect your parts: