The vast majority of people have #1, disposable filters. These filters come in many frame sizes and thicknesses. But you can only use the size that will fit in the spot where your filter goes. So if that spot is designed to accept a 16” x 25”, 1” thick filter, then that is the size you must buy. If you wish to use a larger filter, you must have the ductwork modified to accept that filter size.
There are two major factors to consider when buying disposable filters.
The MERV rating is a measure of how good that filter is at trapping and retaining small particles. The higher the MERV rating is, the smaller the size of the particle it will trap and retain. The problem here is that the higher the MERV rating is, the higher the resistance to airflow will typically be.