Whether you go for a freestanding dishwasher or one that's built-in to your fitted kitchen, you need to measure the space.
How much space have you got?
60cm width or more – You'll be fine with a standard width dishwasher.
45-60cm width – A slimline dishwasher will fit nicely if you haven't got as much space.
Less than 45cm or none – A compact dishwasher is small enough to fit on your kitchen top and big enough to clean up 6 place settings.