Fat Boys by OM in the Krung Thon Plaza (has AC), in the middle of 2F, just off the escalators, has good quality large clothes (shirts, shorts, pants). It is on the north side of Phetchaburi Road (just west of the Amari Watergate) and is connected via the land bridge from Platinum Mall. Regarding MBK, large sizes are available but you should ignore the sizes on the tags, each manufacturer has their own version of sizes. Hold the shirts up against yourself to get a sense.