[Tannaton]

^^^ That's what I was looking for, and is where the petrol can sizes come from :

A portable petrol storage container must—

(a)have a nominal capacity —
(i)no greater than 10 litres if made of plastic; and
(ii)no greater than 20 litres if made of metal;
(b)have a total capacity between 10% and 15% more than the nominal capacity;
(c)be made of either metal or plastic that is suitable and safe for the purpose and will not significantly degrade due to exposure to petrol or naturally occurring ultra-violet radiation;
(d)be designed and constructed so that—
(i)it is reasonably robust and not liable to break under the normal conditions of use;
(ii)the escape of liquid or vapour is prevented;
(iii)petrol can be poured safely from it; and
(iv)it is not unsteady when placed on a flat surface;
(e)be marked or labelled in a legible and indelible form with—
(i)the words “PETROL” and “HIGHLY FLAMMABLE”;
(ii)an appropriate hazard warning sign;
(iii)the nominal capacity in litres; and
(iv)the manufacturer’s name and the date and month of manufacture.

But no mention of how much you can buy?