Hi Quochv,
The best way to start dimensioning your LACs is to have whole BSC(s) in one LAC (may be 1 or 2 or 3) depending upon your signalling..is it 64K or more or less.
If you start with above method, then you can see the possible points where you need to re-parent your sites or possbily increase the sdcch channels for boundary cells to define a better LAC area.
Please revert incase of any clarifications if required.
Best regards,
Garry !!