Will it fit?

An opening that is the wrong size is wrong at every speed, with every sleeve texture and at every price. A stronger motor only makes a poor fit more noticeable.

That makes fit the first thing to check when comparing devices, because it rules options in or out before any other feature matters.

The dimension that decides whether a device works at all is girth, the circumference. Length affects comfort and how much of the stroke is usable, but girth decides whether it fits.

Some specifications are given as girth and some as diameter, and the two differ by a factor of about 3.14. Reading one as the other is the most common way to choose a size that is badly wrong.

If a listing gives a single number without saying which it is, ask the seller before you buy.

One of three things answers the question. A usable internal dimension, measured with the sleeve in place. A stated range from smallest to largest. Or a named sleeve system with published sizes.

Any one of the three is enough to compare devices. A number in a product name often describes something else, so treat it with care.

Sleeve materials stretch, so a usable range really is a range. That is why a stated low and high figure is more useful than a single dimension.

Stretch has limits. A sleeve used beyond its range grips harder, wears faster and can tear at the opening, and a tear usually ends that sleeve. Choosing near the top of a range means choosing a shorter-lived part.

Ask for the usable internal range with the sleeve fitted, smallest and largest, in inches or millimetres, and whether it is girth or diameter.

Ask whether the sleeve can be removed and bought separately, because a removable sleeve turns a poor fit into a replacement purchase. A seller who knows the product can answer all three questions.