When pasting in content from a website you might end up with a single table or a nest of tables (tables inside tables). If you have a nest of tables then the interior content may force the outer table to specific widths and prevent shrinking the outer limits.

But basically, you should be able to select the outer table and right click it to choose Table Properties and set the preferred width to Measure in Percent and then set it to 100%. You should also check its Options to turn off the "Automatically resize to fit contents"

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia