Broadly speaking, there are two types of cucumbers grown: pickling cucumbers, and slicing cucumbers.
Pickling versus slicing Cucumbers
“Pickling” refers to cucumbers that are primarily used for processing or pickling. Pickling cucumbers have thin skins, are short and blocky, and usually have a color gradient from dark green at the stem end to light green at the blossom end.
“Slicing” refers to cucumbers sold fresh for immediate consumption usually as a salad item. Characterized by thick, uniform, dark green skins, slicing cucumbers are longer than pickling types Schultheis, Jonathan et al. Commercial Production of Pickling and Slicing Cucumbers in North Carolina. NC Cooperative Extension Resources. Nov 2014 revision. https://content.ces.ncsu.edu/commercial-production-of-pickling-and-slicing-cucumbers-in-north-carolina/
The pickling ones are grown mostly for canning; the slicing ones are grown for fresh eating.
Slicing cucumbers, which includes English cucumbers, can be used for pickles, but you might get a softer pickle. Slicing cucumbers though always can be used in relish with no hesitation, where any softness won’t matter near as much.
To help, you can make sure to chill cucumbers well before starting.
Pickling cucumber measurements
There are 6 to 7 pickling cucumbers per 500 g / 1 lb.
500 g (1 lb) pickling cucumbers = 1 x ½ litre (US pint) jar of pickles
4 kg pickling cucumbers = 9 x ½ litre (US pint) jar of pickles
6 ½ kg (14 lbs) of pickling cucumbers = 7 litre (US quart) jars of pickles;
1 US bushel of pickling cucumbers = 21 kg (48 lbs)
For gherkins, look for pickling cukes about 4 cm (1 ½ inches) long.
For dill pickles, look for pickling cukes about 10 cm (3 inches) long.
Larger or older pickling cukes are ideal for sliced pickle products such as bread and butter, dill pickle slices, and relishes.
Source: United States Department of Agriculture (USDA). Complete guide to home canning. Agriculture information bulletin No. 539. 2015. Page 6-5.
Trim the ends off cucumbers
Leaving the blossom end on your cucumbers can give you undesirably soft pickles.
“The blossom end contains enzymes that can cause softening. Remove at least 1/16th inch from the blossom end.” Penn State Extension. Crispy Pickles. Blog Posting 19 July 2012. Accessed December 2015.
Enzymes that can cause spoilage are particularly tough to control in cucumbers: “… the natural enzymes in some foods, notably cucumbers, are especially tough. Thus, in making pickles, the heat resistance of enzymes is as much of a concern as the heat resistance of bacteria, molds, and yeasts, all of which are controlled not only by the heat of the processing bath but also by the acid of the vinegar-loaded pickling solution.”  Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 4). Penguin Publishing Group. Kindle Edition.
The blossom end is easy to tell if the cucumbers you have using still have a bit of stem on them: it’s the opposite end from that! However, because otherwise it might be confusing to try to distinguish, many canning recipes just say trim a bit off each end of the cucumber.
If you do trim the end off, the experts at the USDA and its university extensions say that you really shouldn’t need any of the old tricks such as adding a grape leaf, etc., beyond chilling the cucumbers first. Some though suggest you might also want to considering adding Pickle Crisp® (aka Calcium Chloride) to the jars.
Avoid waxed cucumbers
Generally, it is advised to avoid cucumbers with waxed skin when doing pickles — the wax on the skin can interfere with the brine penetrating your cucumber to make it safe.
They are fine to use, though, in recipes that are calling for the cucumbers to be peeled anyway: then, it doesn’t matter what’s on or not on the skin:
Large field cucumbers (as opposed to small, pickling cucumbers) often are sold with wax on the skin to extend their shelf life, whereas the long cucumbers called “English” are sold shrink-wrapped in plastic. For this reason, sometimes more conservative recipe writers will specify English cucumbers. “We don’t peel the cucumber for use in this recipe because the skin brightens up the colour of the relish, making it more visually appealing. Since the peel is left on, we specify the use of English cucumbers. Field cucumbers are often sold waxed to extend shelf life, and the pickling liquid cannot penetrate the wax.”  Kingry, Judi and Lauren Devine. Side note to Chow-Chow Relish Recipe. In: Bernardin Complete Book of Home Preserving. Toronto, Canada: Robert Rose Inc. 2015. Page 221.
|↑1||Schultheis, Jonathan et al. Commercial Production of Pickling and Slicing Cucumbers in North Carolina. NC Cooperative Extension Resources. Nov 2014 revision. https://content.ces.ncsu.edu/commercial-production-of-pickling-and-slicing-cucumbers-in-north-carolina/|
|↑2||Penn State Extension. Crispy Pickles. Blog Posting 19 July 2012. Accessed December 2015.|
|↑3||Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 4). Penguin Publishing Group. Kindle Edition.|
|↑4||Kingry, Judi and Lauren Devine. Side note to Chow-Chow Relish Recipe. In: Bernardin Complete Book of Home Preserving. Toronto, Canada: Robert Rose Inc. 2015. Page 221.|