The joy of software. If you look in this forum you will see posts from users of several different programs, some free, that allow you to model different enclosures. WinISD is one -http://www.linearteam.dk/default.aspx?pageid=winisd
While you can measure the TS numbers yourself the more common way is to use figures published by the seller or available on a forum. You can dump these into any number of software programs that will spit out box size options for sealed systems (the easiest to build or ported systems and other types that are a bit more difficult).
There are also some books and many on-line references as well as folks here and in other forums that will be happy to work with you, if you get into DIY.
And, they won't forget that Thiele-Small is not exactly a household term
. Using software avoids having to wrestle with stuff like this--