Seatposts these days have integrated clamps for the saddle rails, and all commonly available saddles use a standard rail diameter and spacing. Your mileage may vary with very old saddles and super-bleeding-edge-tech ones.
There is no standard diameter for the post itself, since the frame tubes' external and internal diameters, and their treatment during construction, can vary. Size intervals are every 0.2mm and it's tricky to measure accurately at home, though every well-stocked bicycle workshop can measure it for you with specialized tools.
Please do not extend a seatpost past the minimum insertion mark. It's not usually the post that breaks first, though don't risk it, but the frame that will be damaged permanently.
We rarely fail to extract a stuck seatpost, please make an appointment for a shop visit if that's an issue you're facing.
