If you are standing in the filter aisle at Home Depot or Lowe's trying to remember what size you need, here is how to find out without driving back home.
Read the old filter
The fastest method. Pull the current filter out and read the size printed on the cardboard frame. It is a three-number format: width x height x depth. Common residential sizes in Florida are 16x25x1, 20x25x1, 16x20x1, and 20x20x1. Some systems use 4-inch or 5-inch deep filters (16x25x4, 20x25x5) — these go longer between changes and have better filtration at lower pressure drop.
Nominal vs actual size
The size printed on the frame is the nominal size — the rounded number you shop by. The actual filter is about a quarter to half inch smaller in each dimension. A "20x25x1" filter actually measures roughly 19.5 x 24.5 x 0.75 inches. This is intentional. The slight undersize lets the filter slide into the slot without catching or bending.
Order by the nominal size on the frame. Do not measure the filter with a tape measure and order those exact fractional dimensions. You will get the wrong size.
Where to find the filter
Return grille on the wall or ceiling. Many Florida homes have the filter behind the large return air grille in a hallway or living room. Unlatch the grille (it swings open or pulls down) and the filter sits in a slot behind it.
Air handler cabinet. If there is no filter at the return grille, the filter is inside the air handler, usually in a slot on the side or bottom of the unit where the return duct connects. You may need to remove a small panel or slide the filter out of a track.
Both locations. Some systems have filters at the return grille and inside the air handler. If you see a filter in both places, both need replacing. If only one location has a filter and the other has an empty slot, ask your technician which one should hold the filter — running both creates excessive pressure drop.
If there is no size printed on the frame
Measure the slot the filter sits in. Measure width, height, and depth in inches and round up to the nearest whole number. That is your nominal size.
If you are still unsure, bring the old filter to the store or take a photo of it next to a tape measure.