Any bag that complies with the following, dated 2006 but lifted today from the EU Commission website, complies with the "EU regulations", in spite of what a moron with a little petty power might say. Whether or not you argue the toss is up to you!
You are only allowed to take small quantities of liquids in your hand luggage. These liquids must be in individual containers with a maximum capacity of 100 millilitres each. You must pack these containers in one transparent, re-sealable plastic bag of not more than one litre capacity per passenger.
Helpfully, other sources suggest that a bag of 20X20cm would have 1 litre capacity as it would with a uniform depth of 2.5cm, which sounds about right. But there is no regulation specifying those dimensions; all that's needed is is a re-sealable bag of up to 1 litre capacity, regardless of its other dimensions, within reason of course.
Many UK airports have a scam of forcing passengers to buy grossly over-priced "compliant" bags to replace ones which are perfectly compliant. To their credit, BAA does not join in this fraud and gives away bags to the needy, or they did when I was last there (LGW? Can't remember.) In the UK, the racket should be reported to the local Trading Standards office whenever and wherever encountered.