Try in your theme html files. For example:
Code:
themes/Default/html/block-right.html
Change
Code:
<table width="100%" border="0" cellspacing="0" cellpadding="0"><tr><td class="block">
to
Code:
<table width="400px" border="0" cellspacing="0" cellpadding="0"><tr><td class="block">
Might work.