To have any hope of comparing things you need to stick to the same units on every item of comparison. Given the obscurity of what is provided, one might think the producers and retailers are determined to confuse you, although in my experience the volumetric size (in litres of course) has been stated on the moss/compost/etc. pack, but thinking about it I ignore those where I am left in the dark. Although I thought it was a legal requirement to state sizes (in modern units, one would hope), the UK is particularly bad for this and I know that until recently the likes of B&Q and Wickes were selling sand in bags labelled either "Large Bag" or not labelled at all for size - absurd. Even when an attempt is made to give the information, garbled abbreviations appear so one has to ignore a certain level of nonsense. Examples are kgs (=kilogrammeseconds), ltr (not the notation for litre which is l), etc., etc. (oh, gm=grammemetre). It is everywhere, in food (x "portions" - meaningless), energy measurement ("unit", why not tell the humble/thick public you mean kilowatthour=kWh ?). If your "enough to line a 14"/35cm basket" is a quote from the only info provided as to size, then it perfectly illustrates the point - an avoidance of clarity, or in plain English awkwardness for the sake of it.