what is the seatpost size and why does it matter?
seatpost size is usually the diameter of the post, and it matters because there is only one diameter that will fit any particular frame. Common sizes for uni’s are 22.2mm (7/8"), 25.4mm (1"), and 27.2mm.
Sometimes you’ll see people refer to the length as the size, and in that case you just need one long enough to put the saddle in the right spot. Too short and you’ll be on your legs more than you need to. If it’s too long you can cut it down.