Distance for Airport Express?

You "customise" it through the "manual setup" button which allows you to change a variety of different configuration settings. That said, the set of options for extending range is pretty limited. You can change the transmitter power which will definitely alter the range, but this will have defaulted to full power - and hence maximum range - unless you have already reduced it. I guess that turning on interference robustness might increase range if you are in an environment with very high levels of microwave interference - but it will also slow your network down a lot. Changing the channel number might improve range if it is currently running on a channel with a lot of local interference. Assuming that you are not very familiar with WiFi technology (based on your question), make detailed notes of the current configuration first before changing anything - so you can at least get back to working as well as you currently are.

Assuming that the device is running on its factory default configuration, range will generally be far more dependent on environmental factors than configuration. Line of sight range with no walls or other obstructions between the access point and the computer could be a hundred yards or more. A thick stone wall or any significant amount of metal (reinforcing, for example) could reduce this to quite a small number of feet... Subject to the constraints of your house, try to locate it somewhere high and clear.