How much weight/volume do microbes occupy within the human body?
@AlanBoyd's calculations are reasonable, I think the estimate is off though.  The human microbome includes other bacteria which are not necessarily E.coli equivalent.  

The human microbome projects give estimates that microbes are 1-3% total body mass.  i.e. several pounds of bacteria. 

The GI tract alone has most of the microbome mass - faeces is ~60% intestinal flora/fauna by dry weight, which for many adults alone must be hundreds of grams at any given moment.