The values you choose for the capacitors are based upon several factors.
1) What is the specified load capacitance of the crystal.
2) How much capacitance is there associated with the X1 and/or the X2 pins in the microcontroller package.
3) How much capacitance is contributed by the traces of the PC board.

Most crystals are specified for operation with 32 or 39 picofarads (pf). It is typical to find 4 -> 5 pf on IC inputs depending on package style and PC board traces can contribute 1 -> 6 pf if the crystal is placed near the X1 & X2 pins. You typically determine the capacitance to add with the discrete capacitors by starting with the specified crystal load capacitance and then subtracting the sum of the IC pin capacitance and the PC board capacitance. It is common to find values ranging from 18 pf to 33 pf used in different designs.

For your prototype you can start with 27 pf or 30 pf. You will find this to work in almost all cases. Then when you get a final PC board you may decide to determine the optimum capacitor value and select the best standard value for the final circuit configuration. Doing this can bring the actual crystal load capacitance as close the the spec'ed load and thus the crystal will be closest to its printed frequency value.

If the capacitance is off a little it normally just makes the frequency of crystal operation be a little off from its printed frequency value.
Michael Karas