People have many very interesting ways of determining just what they collect.
 Some years ago I exchanged shells with a lady in Puerto Rico who collected
shells that were exactly 3 inches in greatest dimension.  She had clear
plastic boxes of one size, which would just accomodate a 3" shell.  I believe
her cutoff was plus or minus 1 millimeter!  She sent me pictures of her
collection, row after row of identical plastic boxes laid out on glass
shelves, must have been a couple of thousand of them at least.
Another person had obtained several cases of 1" x 1" plastic boxes, and
collected anything that would fit in one of those boxes - tiny shells, shells
just over an inch which would fit diagonally, dwarfs and juveniles of larger
species, etc.  She had thouands of specimens, every one of them in an
identical 1 x 1 box, and all housed in one modest-size cabinet.  Her drawers
(actually shallow trays), were something like 20" wide by 30" deep, so each
tray held something like 600 specimens.  She miniaturized the data slips on
1" x 1" slips of paper, which were in the bottom of the box, under the
cotton, face down, so they could be read through the clear bottom of the box.
Another fellow had a cutoff of 1/4" (about 6 mm).  He collected only
specimens smaller that that.  Again, he had several thousand specimens.
I had one customer who collected orange things - not just shells - a variety
of things, both natural and manmade - but any orange shells I could come up
with were welcome as part of his "orange collection".
Whatever appeals to you is what you should collect.  And folks do!
Regards,
Paul M.
Rhode Island