I've come to like smaller diameter for most grills. N52 Neodymium, 1/4" x 1/4" or 1/4" x 3/8". Sometimes a combo of the two. In most cases, eight would be plenty on smallish grills such as X-LS. Four on grill, Four on cabinet. I also like to put a felt pad over magnet on grill itself to protect finish and facilitate "sliding" the grills off.
Also depends on actual grill weight which can vary some. MDF or ply, NoSeeUm net or traditional grill cloth, etc.
Be conscious of polarity when gluing in place.