Bike size?

I went to look at bicycles for sale today.
They all had stickers with a number underneath the seat.
For example 24 and 26.
What are these numbers for?

Those are the wheel/rim sizes in inch.
24 inch 26 inch.

24 inch wheel and tyres: for young pupils and/or tiny ladies
26 inch wheel and tyres: for young teen, ladies and/or tiny men
27 inch wheel and tyres: for men and ladies

I've come across a few countries that use these measurement types or their bicycle sizing.

Yes,so that number means wheel size. "インチ = inch"(same sound)
Japanese are calling simple just "tire size" instead.
http://www.cycle-hero.com/img/img-size01/img_01.gif
Larger tire is faster than smaller one if same gear using
http://e-bicycle.me-ta-bo.com/images/kyori.png
but need larger parking space and a bit heavier usual.
(but depend on frame weight/material)
http://e-bicycle.me-ta-bo.com/images/Big-small.png

And don't forget "bicycle anti-theft registration".
http://www.nic-nagoya.or.jp/en/e/archives/438
http://www.tokyobybike.com/2013/11/how-to-register-your-bicycle-in-jap...
http://www.ipmu.jp/sites/default/files/webfm/pdfs/secretary/Form%20and...
Also must study about some important rules of traffic.
(no exam/no special license/not so hard)
http://www.keishicho.metro.tokyo.jp/kotsu/jikoboshi/bicycle/menu/leafl...
https://www.npa.go.jp/koutsuu/kikaku/trafficsafety/traffic_safety_en.p...

@ tokyo friend 48
No such recommendations just using a lot like that as market share.
Women loves smaller one is right but now use electric assist heavy one a lot.
BC very very "comfortable" than normal one especially
for up slope,heavy luggage/kids with and against strong head wind
but secret big reason is "won't be muscular legs"
also hate "be running with sweat" especially younger.
However, no good for "diet" or keep healthy body if need :)