Help-how to fix the table width to 800 pix in post new thread

[sellbuydomain]

hi, i have the problem is that, i set the forum size to 800 pixel instead of 100% width. and also i upload it some new simile icons in the forum, now , the problem is the reply thread box is too big, which is not 800 piexl width, for example , check this link
http://www.bw.net/newthread.php?do=newthread&f=2

you see the replied box is 100% width, not 800 pixel width, but the header and footer is 800, so this is so ugly, why is that, why the reply box is expend to 100% , not 800 pix, i set it to entire table is 800, so please help me out this, i don't know which template to modify.