If you want the best performance from the AirPort Express, it will need to connect back to the AirPort Extreme using a permanent wired Ethernet cable connection.
If the Express connects using only wireless, that will reduce the potential speed on the entire network by half or more. So, you will have a "bigger" network, but it will be a slower network. That might still be OK if you do not have a really fast Internet connection.
If you must use wireless for the Express to connect to the Extreme, locate the Express about halfway between the Extreme and the general area where you need more WiFi signal coverage.
For example, if you need a better WiFi signal in the den at the end of the house, locate the Express about halfway between the Extreme and the den.
The more that you you have line-of-sight between the Express and the Extreme, the better the network will operate. Why? Walls, ceilings, furniture, large mirrors, etc will all present obstructions to the WiFi signal, and slow it down even more.
Remember that the Express can only extend the speed of signal that it receives, It cannot "speed up" or "boost" the signal speed.
If you can connect the Express using an Ethernet cable to the Extreme, there is zero loss through the Ethernet cable and the Express gets a full speed signal to extend, not a signal that has already slowed down considerably by the time it reaches the Express.
