Most sizing confusion comes from measuring the wrong thing. The size you need has nothing to do with how large your nose looks from the front.
What the size actually refers to
A strip works by spanning the nasal valve — the narrow soft section about a centimetre inside the nostril where the sidewall draws inward on inhalation — and pulling those sidewalls apart.
The correct size is the one whose ends land on the fleshy sidewalls of your nose, on either side of that soft section, with the body of the strip across the soft part just below where the bone of the bridge ends.
So the relevant measurement is the span between those two anchor points. Not nose length, not nose height, not perceived size.
Find the anchor points
Run a fingertip down the bridge from between your eyes. You will feel firm bone, then, roughly two thirds of the way down, the surface gives and turns soft. That transition is the top of the zone.
Now feel outward from there. On each side, a centimetre or so out, the nose becomes flesh rather than cartilage or bone. Those two points are where the ends of the strip need to sit.
Measure between them if you want a number. That is your span.
Too big versus too small
Too big: the ends extend past the fleshy sidewalls onto the cheeks. The spring is then pulling against skin that is anchored to bone and does not move, so it produces no lift. You also get more adhesive on your face than necessary and a strip more likely to peel at the corners overnight.
Too small: the ends land on the bridge or on the nostril rims. On the bridge, the spring pulls against bone — nothing moves. On the rims, it pulls the flexible nostril edge, which flexes without opening the valve, and lifts within an hour or two.
Both failure modes look identical from the outside: a strip that is on your face and doing nothing.
The test that settles it
Skip the measuring if you like. A correctly sized and placed strip produces a difference you feel within seconds, awake, sitting on the edge of the bed. Inhaling should be noticeably easier and you should feel a mild outward pull.
If you feel nothing, try a different position first — a few millimetres higher or lower — before concluding it is a size problem. Placement is the more common error by a wide margin.
If two or three placements at the right size produce nothing, your restriction is not at the nasal valve, and no size will help. (The ten-second test for where your restriction actually is.)
Practical guidance
Most adults fit a standard size. Brands offering small and large are usually accommodating children and teenagers at one end and unusually broad noses at the other. Reaching for large because you think of your nose as big is a common and unhelpful instinct.
If you are between sizes, go smaller. A strip that sits entirely within the soft zone works. A strip that overshoots onto immobile skin does not.
Do not cut a strip down. The spring bands run the full length and are tensioned as a unit; cutting one severs the mechanism and you are left with adhesive.
Children need children's sizing — an adult strip on a small nose overshoots the zone completely. And a child who snores regularly needs a doctor rather than a strip. (Why.)
The variable that matters more than size
Band strength. The lift is the mechanism, so a correctly sized strip with a weak or absent spring is still decoration.
TitanAir strips are built around spring-loaded bands producing genuine outward pull, which is the actual difference between a strip that works and a flat adhesive band that sits there. (Titan publishes the material testing.)
Then get placement right, which decides more than either. (Where it goes, step by step.)
The bottom line
Size means the span between the fleshy sidewalls either side of the soft section below the bone — not how big your nose is.
Most adults fit standard. If in doubt go smaller, never cut a strip, and remember that the immediate awake test tells you more in five seconds than any measurement will.