What size can l get?
And how to fit it?
Where is the cheapest to get it?

Easy.

Size, its a standard din size unit, so any standard size head unit will fit.

How, using the stereo removal tools (cheap from Citroen if u do not have any) insert the tools into the little holes on either side of the stereo & pull. This will pull the stereo out of the dash, then just disconnect the aerial plug & the 2 power & speaker plugs from the rear of the set.
Refit the new stereo by plugging in the connectors & just pushing it into the dash until it clicks in place.

Cheapest, there r hundreds of different brands & specs of head unit ranging from less than £10 to more than £2000, so u will have to do some hard work searching as only u know want u want.