Q: What is the range in meter of the airport êxtreme and express

A precise answer is impossible, since a number of variables affect wireless installations. At one time long ago, Apple used to publish range specifications. They no longer do, for this very reason.

Here is what they say today:

Actual performance will vary based on range, connection rate, site conditions, size of network, and other factors. Range will vary with site conditions.

... this itself is odd, since it implies that they at least provide an estimate of range, which they do not.

In ideal conditions with no interference, intervening objects, or competing wireless networks, you can expect a maximum range of about 100m. The presence of any of these sources of interference will reduce this range. If you live in an area with many other wireless networks, or your AirPort is behind several brick or stone walls, you may only get 15 - 20 m, perhaps even less.

The only way to determine its actual range is to use one.

There are two ways to "extend" the range of your AirPort network - a "wirelessly extended" network, and a "roaming" network. 

Roaming is better.

Read about each of them here:

Wi-Fi base stations: Setting up and configuring an extended wireless network (802.11n)

Wi-Fi base stations: Setting up and configuring a roaming network (802.11 a/b/g/n)

You might get a usable signal from your bedroom window. Place the Extreme or Express right on the window sill, within sight of your bus stop.

Buy the Express.

Do not buy an Extreme thinking it will give you more range than an Express. The transmitter power is identical for both.

If you want to create an extended network then you must use only Apple AirPort base stations.

Nico20 wrote:

 

So I can't expend my non-apple wi-fi?? Whit an Apple AirPort express ...

No! Only Apple AirPort base stations can take advantage of Apple's "extend" feature.

Read: Wi-Fi base stations: Extending the range of your wireless network by adding additional Wi-Fi base stations

Definitions

 

Wi–Fi base station - Any variety of AirPort Extreme Base Station, AirPort Express, or Time Capsule.

The only way to "extend the range" of a non-Apple base station by using an Apple base station is to create a "roaming network".