The devil is in the details or is it? Well the answer is a big fat YES, every light fitting has a size and you need to get the details before you buy, you will need some basic measurements and a tape measure.
So look at your space if there is a light hanging there already thats great bonus, so measure it, you want the drop which is the height from the ceiling to the lowest part of the fitting. next measure the diameter which is the width of the light fitting. These two things give you the first point of scale and seeing something hanging in a room already tells you if you want it bigger or smaller than these measurements.
If you do not have an existing light fitting, due to a new build or renovation then get a step ladder (do be careful) and measure from the ceiling to the place you would like the light fitting to end, again this is referred to as the drop or overall height of the light fitting.
Industry standard recommends that you allow 6ft 10 inches between the floor and the base of the light fitting which allows movement around the place safely, it does not have to be as low or as high as the recommended as the design is yours to do with as you would like.... A room scheme can be fun to design and some people will put a long light in a corner or in series as a room divider there is no hard rule just a guideline.
Next you still need to think about the width of the light I have advised many people to hold up a bucket or an umbrella even a golf umbrella to give you an idea of the width of a light. Some people love a very wide light even in a room with a low ceiling as you can buy numerous fittings that are shallow but wide.
So.... every light sold online and in a book at a store should have these two measurements, height and width, some go a little further and tell you the height of the fitting, again this can be important if you have a high ceiling as you don't want the light way up high you may want it hanging down a bit further and this measure allows you to decide the start and end of the light plus how much chain and cable will be above it. Many people confuse the height of the fitting with the overall drop including the fitting so do take a moment to consider the figures that you are reading.
The only other measurement is that of light output and if you presently have a fitting in the room, consider if is it bright enough with the number of bulbs and wattage you are using or do need a little more or less, these questions you can raise with friends and family to see how much light you need, those who are partially sighted or ageing need a bit more light than a younger audience.
Measuring for a light is not a hard thing to do but it is essential and to be honest it frustrates me enormously when a customer returns an item because they have not measured first!! If you are lucky enough to get a refund or exchange then its not too big an error but if the fitting has been made to order than the problem is yours to own so remember measure, measure and measure again.....