So far as the commercial product is concerned:

PICKLE = CUCUMBER

Other things (onions, beets, okra, etc) are sometimes picled, but the "little green weenies" are mostly cucumbers.

I don't know if it remains true, but in most parts of the US the State Agriculture Agent was, a decade or two ago, one of the better places to get advice on pickling and other "canning" processes, since their business is food safety. 4H and FFA orgs might also be helpful, and if you find an active group they may have lots of other helpful "domestic" advice at hand.

Custom in my area when I was a kid was to select the small ones to pickle whole. The art with the little ones is getting a "crisp" pickle - if that's how you like 'em.

Larger ones were sometimes "quartered lengthwise" but more often just sliced - for better penetration of the juices.

The little ones usually were made up as "sour pickles" but the slices were usually finished with a fair bit of dill and a touch of clove, mild and "sweet" and called "bread & butter" pickles among my crowd. Most people mixed some "onion rings" in with the B&B slices, and a few people liked to add some slices of red bell pepper just for the color.

If you have a recipe for pickling onions it's probably quite similar to what you'd do for the milder cuke pickles, although cucumber pickles more often use some alum along with the salt for the "presoak" brine.

John