Malls that carry petites:

Copley/Prudential Mall in Boston - Ann Taylor, Banana Republic, and LOFT - each with a petites section, but Ann Taylor does not carry 00P nor XXSP. Also a White House Black Market with size 00/xxs, and an Armani exchange with size 0p.

Chestnut Hill Mall in Brookline - A small mall with a large Banana Republic petites selection, some Ann Taylor petites, a Cole Haan, Bloomingdales, Bebe, and Victoria's Secret

H&M Downtown Crossing - A big H&M, usually with plenty of tiny size 2's.

The Chestnut Hill Mall is definitely a must-see for any petite! Banana Republic has a dedicated petites store in this mall! (It's very fun going into a store where EVERYTHING is petite!) If you catch a good sale and factor in the lack of MA sales tax, you can get some good deals here.

I'd also suggest that petites keep an eye on high-end consignment stores in the area. The stores don't have dedicated petites sections, but I have often scored a great petite or vintage find at Second Time Around (various locations, but I recommend the Beacon Hill or Newbury St. locations) and Poor Little Rich Girl (the Boston neighborhood of Somerville).

Arsenal Mall in Watertown is great for bargain shopping. Ann Taylor Factory Store (they do carry petite sizes, and have great sales), Marshalls, Forever 21, Gap Outlet, etc.

Personally, I like CambridgeSide Galleria. There's an H&M, Ann Taylor (although kinda small), Banana Republic, Gap, and more recently a TJ Maxx.