The Xsara accepts a standard DIN size radio unit, however to fit any radio other than the standard Xsara one (including other Citroen radios and aftermarket ones) will require the use of a new facia plate to go around the radio and blank off the mounting holes which will be left exposed if you use a different set.
Do not use a radio that is designed to fit a mulitplexed Citroen as they require digitial communications with the BSI before they can be correctly fitted, if you try it the radio will just emit a loud annoying beep every few seconds until you program it to the BSI (which you can't do as your car is non multiplexed and so doesn't have a BSI).
Multiplexed Citroens are - C2, C3, C4, C5, C6, C8, Xsara Picasso '02>, C4 Picasso, Berlingo '02>, Multispace '02>, Relay 3, Dispatch 3, Nemo, Xsara '02>.
Citroens that have radios that would fit and work fine are - Berlino '51<, Xantia, Saxo, Xsara '51<. The standard ZX radios were often double DIN size so be careful with them.
Citroen Senior tech.