dear i want to know if there is a different between two crystals small one and large size one (xt) or they are the same with PIC.
As far as i am aware, as far as a PIC is concerned, any crystal (4mhz or 20mhz generally) will do. just go for the cheapest really
Do you mean crystal oscillator? Or do you just mean a crystal. If you're talking about a crystal oscillator, so long as it's the 5V type, it will work fine. If you're talking about a crystal, make sure it's of the parallel cut type and that you don't overdrive the crystal if it's a type with a low level drive spec.
There are two standard sizes in wired crystals, the tall HC-49/U case and the low-profile HC-49/S. No big difference AFAICS, the low profile is a convenience where it helps keep board height down.
