This is a relatively simple task to do. Consider the size of your sofa or couch and consider your taste and comfort. In the former case, if you have a large couch, it is safe to buy large throw pillows.
But when the sofa is compact, it is always better to go for smaller pillows. Small pillows are also the best options when it comes to armchairs. The next thing you need to consider when deciding on your preferred cushion size is your comfort-factor. If you are a person who likes to snuggle up between fluffy cushions, then you might as well get the big fluffy ones. But if you are someone who likes to keep the space constraint in mind, then you might want to go with the ‘small-for-small’ rule. Having larger cushions, (although comfortable) might turn into a headache when you end up having to remove them every time there is an extra guest.