With a tape measure, Caran. They will also take into account the accessibility (do they need to work off a plank, etc) of the area that needs to be painted and any prep work that needs doing. Your best bet would be to try and get someone who will give you a free quote. Then, even if you cannot (or will not) meet his price, you can always use his measurements when contacting the next decorator on the list. Better still, do you not know someone who knows someone who knows someone who will do a decent job for a bit of spending money. Nudge, nudge, wink, wink.