Diameter is the width of the filter, in millimetres. In this shop the numbers that appear are 13 mm, 25 mm, and 33 mm.
13 mm is the smaller disc. 25 mm is the one most titles mention next to it. 33 mm appears on fewer listings, including the 5/10/25-piece pack and the 10-piece pack. A 13 mm option on one product does not mean that product also offers 33 mm. Read the title, then the option you select.
Diameter and pore size sit next to each other and are easy to swap. 25 mm is not 0.25 µm. 13 mm is not a pore. If you are matching a holder or a syringe you already use, match the millimetre number, then check the pore on its own line.
The shop can list just the 25 mm packs, or 13 mm, or 33 mm. Start there if the width is the part you already know.