I Can't Do This
Yes, you can do this. By "this" we mean you can absolutely measure a filter. It is easy and saying your A-B-C's. All you need is a tape measure, pen, and paper.
Let's start. Grab your tape measure and start on the side to measure the height/length. Once you have that number write it down as your Length (L).
Next measure across the top to find your Width (W) and that down. Then measure the Depth (D) of the filter and then write that down.
Now you should have your size. It will look something like this example 23.5x23.5x1.75" you will not have exact number measurements. The measurements you will have is what we call the actual size while most manufacturing companies will use a nominal size rounding up to the nearest whole number. So based on the example numbers above you would want to purchase a 24x24x2 Filter.
It is as simple as that. Don't say "I can't do this," because you can.