There are really clear rules on BA.com about the size of handbaggage.

Please don't bring on any bags that are larger than the size quoted in these pages.

If you bring on a second bag, please be aware that this has to fit under the seat in front of you (not if you are in a bulkhead or exit row though). If you don't like the idea of having a bag under the seat in front, don't choose to bring on two bags.

http://www.britishairways.com/en-gb/...age-allowances

Please abide by these incredibly generous allowances or don't complain if they get more restrictive which they will if you all actively encourage each other the abuse the rules.