Txts are sent to firstname.lastname@example.org
A txt sent to this address would arrive in the following format (tested using lj's send a text thingy):
email address: (f:username) message
So the lj email address would use up 29 characters (including the : and space).
The username would take up usernamelength+5 characters (including the space at the end).
Which leaves 160-29-5-usernamelength for the txt. So, if we take thevoicewithin as the username we end up with 126 characters. None too shabby.
If you need any other info please ask.