Measure once, and then measure again. The most important thing about your kitchen island is that it fits, without getting in your way as you move around the kitchen - ideally, you want around 42 inches of walking space on all sides. Designer Christopher Peacock even recommends the island itself should be no wider than five feet, and no longer than eight, as this allows you to reach across it easily. Whichever size you go for though, the island should enhance your kitchen, not shrink it; if you're not sure, pick one you like and pile up some cardboard boxes to the same size and shape in the place you have planned for it. Live with it for a few days - you'll soon work out if it's not the right fit!
Think carefully about what you're planning to use your kitchen island for, as this will significantly impact your decision. You might want to use it as a dining area, in which case it's a good idea to line the outer side with chairs and ensure that the countertop is wide and that it has rounded edges for diners to rest their arms on while eating. Cupboards or shelves on the kitchen side of the island are a great idea for extra storage, and you could even install power sockets and use the outer side as a work space, ideal for children or even just for breaks between cooking.
If you plan to use it as a cooking centre, consider all of the different appliances you can include. An oven and hob can be fitted with an attractive extractor, although this will require quite a deep island to ensure nobody can accidentally reach the hob from the other side. Try to ensure that your workspace faces outwards, so that you can still socialise while preparing meals.
Style and Design
A purpose-built kitchen island is the most flexible option, giving you the chance to determine whether you want storage space, how the countertops and cabinetry match up with your existing design, and any other additional designs. It's a great way to add the things you always wanted, like a wine rack or a cookery book holder. They come in plenty of different shapes and sizes, too, so take a look at oval and kidney-shaped options as well as the standard oblong.
Other options vary quite a lot, but are worth bearing in mind. If you have a small kitchen you might find a roll-out island helpful, as you can tuck it away again when you're not using it, and use the storage underneath to hide appliances or items you rarely need. Try to bear your design scheme in mind; the most popular option is to match the island to your current cabinetry, but you might find that contrasting colours work just as well, and turn your kitchen island into a feature piece.