Dishwashers come as small as 18" across, but most are 24". Make sure to measure, including the hallway, to ensure you get the right size.
If you're low on space a portable dishwasher is a great option. For full-sized kitchens, built-in is best.
ENERGY STAR models are 10% more energy efficient on average due to more efficient water jets & racks designed to maximize performance.
Look for handy features such as adjustable racks, filters, delayed start & quiet operation.