If you want to measure the length of your shed then you will need to drill a small hole in each end. Then hire the Large Hadron Collider for just one day (or maybe two just to be safe), and position your shed in the middle of tunnel. Now when the protons get up to an energy of 7 TeV time how long it takes each proton to go in one hole and come out the other - I have always found Casio watches to be reliable. Since the protons will be travelling at 0.999999991c it is an easy matter to work out the length of your shed (incidentally, Casio make some rather super calculators too!).
Of course that depends on whether you can borrow a drill or not.
Has the Large Hadron Collider got a plug on the end and if so will lindylou be able to use an extension lead to plug it in through the kitchen window (if she doesn't have a socket in the garden - I presume she couldn't plug it into a socket in the shed itself because of the vacuum)?
at the risk of being a spoilsport, I don't think sheds have been decimilised - they still come in imperial i.e. a 5 by 7, 6 by 8 or 7 by 9.
Are you trying to find out what size you've got, what size you want to buy, or the size of shed you need based on how much stuff you have or the size of the space you have to fit a shed in?
I don't really want to bring Health and Safety into it but she should also get one of those circuit breaker things just to be on the safe side when she orders the travel plug adaptor - boring - but safety is paramount!!