Insert a table into an empty page in FrontPage.
Right click in the table in design view and left click on Table Properties.
In the pop-up dialog box, specify the table width as 760 pixels for a fixed width or select 80% to have the table resize depending on the screen resolution of the site visitor.
Just to the left of the fixed width settings you can choose to set Alignment to Centre from the drop down list of options.
Now, if you added a page backround colour or image but left the table backgroud white you would end up with an interesting contrast of colour with space between the edge of the page and the browser window the page is in.
There are other ways but this is about the simplest.
80% table width gives you a 10% gap at left and right while specifying a fixed pixel size may also work well, depending on your exact intentions.