If you are buying footwear and you have swollen feet, you will need to look for wide fitting shoes. They will offer you more depth and it shouldn’t be necessary to buy a larger shoe size.
It’s also important to choose the right time of day to try footwear on unless the swelling is well controlled. Early morning is not ideal as your feet may have only very moderate swelling and late in the day your feet are likely to be at their most swollen. It is best to try on new footwear around the middle of the day to get the most accurate fit.
If you wear support hosiery then wear it when you are going to try on shoes. If you need to wear orthotics, take them with you so you can check that the shoes will accommodate them.