Most heating and cooling systems will have air filters in two places; in the furnace mechanical unit, or in the return air vent inside the house. You will need to open those compartments and pull out the existing filter and the size should be stamped on the filter. If you can’t read it, you can simply take a tape measure and measure the 3 dimensions – length, width, and thickness (i.e. 20”x20”x1”).
Dryer exhaust vent ducts (the big silver metal duct in back of dryer that leads to wall) should be checked once per year. Even if you regularly clean out the lint trap in the dryer, lint and debris build-up can still occur through the dryer exhaust and will build up in the exhaust vent duct. This duct should be cleaned out when inspected to avoid any potential fire hazards.