Table and Cell Width

A table can be defined either as a absolute figure or as a % of the total width. If you set it as a % figure, 100% will fill all the space available on the browser screen. This will mean that it will always fill the screen at different resolutions.
You can set this up by selecting the table and altering the setup in Standard View from the Property Inspector.

Cells can also be defined as a % figure. You cannot do this in the Property Inspector. Instead select the table and gothe Modify/Table menu to change to %. This will convert the cells too. Alternatively go to the code and alter it there.

From Dreamweaver Help......Setting column width

A column in Layout view can have either a fixed width or a width that automatically expands to fill as much of the browser window as possible (“autostretch”). Information about the width appears in the column header area at the top of each column of the selected table, if layout table tabs are showing (see Setting Layout view preferences).
A fixed-width column has a specific numeric width, such as 300 pixels; the width appears in the column header area (unless the column is too narrow for the numbers to be displayed). An autostretch column’s width changes automatically depending on the browser window’s width; the column header area for an autostretch column shows a wavy line instead of a number. If your layout includes an autostretch column, the layout always fills the entire width of the visitor’s browser window.
Note: The width you specify for a column applies to all the cells in that column.

You can make only one column in a given layout table autostretch. A common layout is to make the column containing the main content of the page autostretch, which automatically sets all the other columns to fixed width.
For example, suppose your layout has a wide image on the left side of the page and a column of text on the right. You might set the left column to a fixed width and make the sidebar area autostretch.

When you make a column autostretch, Dreamweaver inserts spacer images in the fixed width columns to ensure that those columns stay as wide as they should be, unless you specify that no spacer image should be used. A spacer image is a transparent image, used to control spacing, that is not visible in the browser window.