It should be:
<td width="200">
or
<td style="width: 200px">
Note that if your cell contains some content that doesn’t fit into the 200px (like somelongwordwithoutanyspaces), the cell will stretch nevertheless, unless your CSS contains table-layout: fixed for the table.
EDIT
As kristina childs noted on her answer, you should avoid both the width attribute and using inline CSS (with the style attribute). It’s a good practice to separate style and structure as much as possible.