I don't believe that Thunderbird asserts any particular limits. It may suggest that you use its filelink feature to send large files, but this is not compulsory, and IIRC you can set the file size to trigger this reminder yourself. There may be limits associated with the data types in use, and these may be a function of your Operating System or the compiler setting used when building Thunderbird. The most significant and most likely limits you will encounter are set by the various servers your messages pass through. You'll have to ask your email provider, ISP or your correspondents about these limits.