There isn't a simple answer here. Basically, you'll want a speaker that's going to be able to fill your intended space with sound. So, if you're looking for a small, portable Bluetooth speaker to use casually on the table outside when you're hanging out, a relatively low-powered speaker with a 30 to 50-watt output will get that job done for you without any problems.
However, if you've got a big, spacious room that you want to fill with sound at decent volumes, a bigger speaker with a higher watt output will likely serve you best.