The SIA query tool is meant to be used to get cutouts of the given size around the specified point, i.e. it is asking the question "which CCDs in the table contain this point?" By varying the size parameter you are changing the size of the cutout to be generated (clipped to CCD boundaries) but the number of CCDs containing the query position will be the same unless/until new images are added to the database. I have verified that in the archive image table there are only 15 raw g-band images containing this position.
If you instead you want to know how many CCDs are within a specified radius of a position you can query the image table using a the Query Interface for the TAP service with a query such as the below where the '2' in the q3c_radial_query() is the search radius in degrees. Increasing this value will increase the number of images found.
Hope this helps, please let us know if you still have questions.
WHERE prodtype='image' and calib_level=1 and obs_bandpass='g'
and 't'=q3c_radial_query(s_ra, s_dec, 43.20, -66.50, 2);