In this category size is specification, and the good news is that much of it is standardised. The bad news is that sellers quote the brand and skip the number.
Cricket bats
Size has standard names — the junior sizes by number, then harrow, then short handle and long handle for adults. Ask which, and ask the weight, because two bats the same size can feel completely different.
Rackets
Grip size, usually numbered. Head size in square inches or centimetres. Unstrung weight or weight class, and balance if you care about it.
A racket in the wrong grip is the wrong racket, however good it is.
Shoes and boots
Inside length in centimetres, not the printed size. Sports shoes differ across makers more than ordinary shoes.
Gloves, pads and protective gear
Sized by hand or limb measurement, and the maker's chart differs from the next maker's. Ask for the size marked and a measurement of the item.
Weights and bars
Bore diameter of the plates, which decides whether they fit your bar at all. And the actual weight — stated weights are frequently the label's rather than the item's.
What to ask the seller
"What is the specification exactly as marked, and can you measure it?" Questions to ask.
What protection covers
A specification materially wrong or a different model than described: seven days from delivery. Gear that was accurately specified and does not suit you is not.
Questions people ask
Can a bat be made lighter?
A little, by a specialist, and it affects the bat. Buy the weight you want.
Does grip size matter that much?
Yes. It is the most commonly ignored specification in this category.
Are children's sizes consistent?
Less so than adult ones. Measure.