1) Be sure to follow our installation Tutorial Videos here:http://www.shape5.com/joomla_tutorials.html
Note that a Site Shaper cannot be installed to a pre-existing Joomla Installation.
2) In regards to this error:
Warning: POST Content-Length of 17374307 bytes exceeds the limit of 8388608 bytes in Unknown on line 0
It sounds like your PHP settings for Max Post Size on the server need to be increased. Changing this depends on the software you are using but generally it will be in the php.ini file under post_max_size. Just change this to a larger value to allow for installation.