It depends on how the data is actualy stored. This calcualtion only holds true for char and nchar data types, and is based on SQL Server. If fusetalk uses an image, or text data type, then this can be MUCH larger, but the end result is roughly equivelent to the actual text of the post.
varchar or nvarchar: 23168191 * 8000 * 4 = 74138211200 (This post that I am posting in is post number 23168191 if you were wondering)
So, uncompressed, the text that makes up all the posts ever posted would be 74.138 GB.
Straight text compresses VERY well, so assuming what I can compress text fields that are the same size at work with gzip would make 74.138 GB of text be about 7.4 GB.