Outdoor gear is sized in names that sound like promises and are closer to hints.
Tents
A "person" rating is nominal. It describes how many sleeping mats fit side by side with no room for packs, boots or anybody turning over. Most people treat a two-person tent as comfortable for one.
Ask for floor dimensions, peak height and packed size rather than the rating. And ask the packed weight, if you are carrying it.
Sleeping bags
Length — bags come in regular and long, and a bag that is too short is cold at the feet all night. Shoulder girth, which decides whether you can move. Packed size and weight.
Temperature ratings are a separate thing and makers differ in how they state them. Ask what the bag is marked with rather than what it "should" be good for.
Backpacks
Torso length, which is the measurement that actually decides fit — not your height. Many packs come in back lengths, and the wrong one hurts regardless of how good the pack is.
Then capacity in litres, and whether the hip belt is sized or adjustable.
Boots
Inside length in centimetres, and ask about width if you know you need it. Walking boots are worn with thicker socks, which changes the fit you want.
What to ask the seller
"Can you give the actual dimensions rather than the rating?" Questions to ask.
What protection covers
Dimensions materially wrong or a different model: seven days from delivery. A tent that is accurately described and smaller than you imagined is not.
Questions people ask
Is a three-person tent right for two?
Frequently, if you want room for packs.
How do I measure torso length?
From the bony bump at the base of your neck to the top of your hip bones.
Do bag temperature ratings compare across makers?
Not reliably. Ask what is marked.
More: buying outdoor gear, storing it.