I want to check how much storage space my emails are taking up

That is such a simple question. The answer is not. You can check the size of any folder by right clicking it and selecting properties. But your Thunderbird profile size will be a multiple of the actual size of your mail. There is a little dialog that pops up when you first run Thunderbird that offers to allow windows search top search your email. Checking that makes of copy of every email, of the first 40kb or so of it. You don't get told that and most folk just click the box because it sounds like something that might be useful. I unchecked it is setting some years ago and then had to manually delete over 100,000 files that were created using it. I never search email with Windows search, I use Thunderbird for that., Then there is Thunderbird search itself. It uses a global database which is essentially a huge database of email texts and can be gigabytes in size. This article discusses it somewhat https://support.mozilla.org/en-US/kb/rebuilding-global-database When using Windows you can open the windows file manager and type %appdata% into the toolar and select the Thunderbird folder that appears. Right clicking it will offer a properties option that will show you the amount of storage the profile takes.