It's not clear what you're asking. If you'd like to take a list in which each line contains two words and convert it to a table that contains two columns -- one for each word -- then you could easily do this by selecting the entire list, click Insert > Table > Convert Text to Table; in the Separate text at section of the dialog box, add a space in the Other box, and click OK. (Before converting, make sure there are no extra spaces at the end of each line, or Word will create a blank column.)

Or are you asking how to create a table that spans exactly the entire width of a page, but no more or less? If that's the case, then you can right-click on the table and, via Table Properties, set the Preferred width to 100%.