There's no definitive answer to this question as it depends on a number of factors, such as the size of your yacht, the number of people you want to accommodate, and your budget. However, we can provide some general guidance to help you choose the right sized dinghy for your needs.
If you're looking for a dinghy that can be used for tendering (i.e. ferrying people back and forth from shore to your yacht), then you'll want something that's stable and spacious enough to comfortably seat several people. In this case, a good rule of thumb is to choose a dinghy that's at least 10 feet long.
If you're mostly interested in using your dinghy for recreation (e.g. fishing, swimming, etc.), then you might be able to get away with something smaller and more maneuverable. In this instance, an inflatable kayak or SUP might be a good option.
Ultimately, the best way to determine what size dinghy is right for you is to consult with a professional or try out different options before making a purchase. This will help ensure that you end up with a vessel that meets all your needs and expectations