Well, that is a problem I've been through in the past, specially when using Dreamweaver 3. But in DW4, this problem seems to be solved as, it is more WYSIWYG than 3, and one extra reason for this, is that you can work not only in standard view, but also in layout view. Working in the last mentioned layout, you can't go wrong about what you are trying to achieve. But also in standard view, at least in DW4, I had never have problems when creating such borders.
BUT, if you are using DW3, and cannot switch to DW4, or using DW4, but still have problems, I suggest trying to create a simple one-cell table, with the size you want, and then inserting in that table the table which include your borders, and assign it to have a 100% size inside the single table.
Didn't worked though? well... there is something more to do, although may complex things but only in code view:
Assuming you've done the above (putting the desired table in the sigle cell-table), try creating a table inside the the borders' table (That means, put all the contents in the new last created table, and just leave only the borders to surround the contents' table), put all the data itside that table, and also give this centered table a size of 100%.
I hope this help. If my description is not good for you to understand (maybe I'm not so good in giving someone a solution to understand), feel free to ask anything you want including some wonders you have about my solution, or some clarification too.
if you didn't get bored reading my little solution...
