Did you know that your Messages app is set to keep all messages forever? Yes, this is the default, and over the years your texts can take up a massive amount of space not only on your Mac, but also your iPhone, and iPad.
You can have Messages delete any texts that are more than 30 days or over one year old automatically. On the iPhone and iPad you can go into Settings -> Messages and scroll down to Message History. Here you can choose to keep messages Forever, One Year, or 30 days. On the Mac in the Messages app select the Messages drop-down menu and Preferences. Choose the General tab and in the middle of the page will be the option to choose how long you want to retain your messages.
One thing to note: If you add a new device to iCloud Messages, it will toggle back to Keep Forever. This is annoying, but really, would we expect anything different from technology? And to keep that technology happy, it's best to make sure that whatever retention frequency you choose, you do the same across all of your devices.