Food processors are typically the most expensive small appliance for your kitchen. If you are a serious cook or planning to be one, a food processor is a must. They are capable of slicing, grating, chopping, pureeing and even kneading. They can vary from a very small (mini) food processors to machines with a 16 cups capacity that can handle all the food preparation for a large dinner party.
It's measured in cups and ranges from 4-16 cups. Having a bigger unit would probably be the best because you never know when you'll have to use it for a large party, but if limited space to store it then a smaller unit might be more practical. Smaller machines are easier to clean, but they are obviously limited in the capacity. Just make sure when you clean the unit to watch out for the very sharp blades and not to leave it around children.
Power is measured in watts and ranges from 500 to 1200 watts. The more powerful the machine is the more expensive it becomes. But usually more power will give you better results.
Food processors are supplied with a set of attachments for shredding, slicing, whisking, blending, citrus juicing, mixing dough or grinding. Usually price dictates what (and how many) attachments will come with the unit. Many models come with a storage case which you can store all the attachments. Also, they might include a spatula and the recipe book. Make sure you will use all the attachments because if you don't, they will just add clutter in your drawer.
Bowl sizes range from 4 to 16 cups. They can be made from a plastic material, which tends to be cheaper or they can be made from stainless steel, which lasts much longer, but it's also little pricier. Some food processors come with more than one bowl size. This can be a very nice feature especially if you are making something smaller and you don't want to clean the big bowl.
Most food processors have this function. This function ensures that the processing cannot start unless the lid is locked on properly which prevents any kind of accidents.