Re: lower limits to BSB/SSA/REC is related to being a 757, I believe. Apparently always enforced to oversized/overweight boxes but usually also to bags and also quantity of pieces (never flown these routes, information sourced from a friend who lives in Salvador)

As for United, my last trip with them (IAD-GIG, last year, Business in a 3-class 777), I had no problems checking 3 bags. Never flown from IAH.