Mica: A type of dielectric. Mica is inert it will not change over time or temperature, and is used in high precession applications. Mica dielectric capacitors are covered in the MIL-PRF-39001 specification.
The graphic below shows how Mica capacitors are marked per the Military Specification. A commercial mica cap might just be marked with the value [in pica-farads], the tolerance, the working voltage, and the part number.
Mica Capacitor Part Marking
The graphic indicates how to read a Mica capacitor.
The physical size is determined by the capacitance and working voltage, which is why the picture of a Mica capacitor above has no dimensions [other than lead length]. However as a general rule a Mica capacitor will be larger than a ceramic capacitor for example.
Mica capacitors would be found in precession analog circuits, and never in a digital circuit.
Of course the graphic shows the classic mica dielectric body style, but there other through-hole styles and surface mount types. Some common capacitor chip sizes include; 0805, 1210, 1812, and 2220 body sizes, having values between 0.5pF and 2200pF with working voltages between 100 volts and 1000 volts. A mica dielectric capacitor will be more expensive than other capacitor types. Although much of this page deals with capacitor marking, there my not be much if anything marked on a surface mount capacitor. Maybe a part value in pF, but nothing more than that.