Most filters are measured and sold using their nominal dimensions.  The nominal dimensions are an approximation of the actual dimensions of the filter.  The filters in the following example are nominally equivalent:

Note: the length and width are interchangeable, so a 16" x 25" is the same as a 25" x 16".

Look For The Nominal Dimensions On Your Filter

Your filter probably has the nominal dimensions printed somewhere on the frame.  Look for them and use them to find a replacement filter.  If your filter does not have any visible nominal dimensions, you should measure your filter and round each actual dimension up to the nearest inch.