The new generation of wireless keyboards would are using new technology that offers full encryption of the signal and authenticatification with the other device, using different available channels for the signal, so only your receiver will read your keystrokes.

Wireless keyboards should have a theoretical radius of about 20-30 meters if we assume a clear path from keyboard to receiver.

If you buy a quality keyboard the chances that your keystrokes would be seen by somebody else are very very low.