Resize the contentPlaceHolder in the Master Page?

"How to resize the contentPlaceHolder in the Master Page?"

The content place holder size it determined by the size of the content in your content pages. If you look at the properties for the content placeholder, you will see that there are not size properties.

If you want to size the contents, try wrapping your inner content with a table of a particular size.
Here is another solution:
Insert the ContentPlaceHolder in a DIV tag and use the Style attribute to position as you wish: 
<div style="left: 100px; width: 100px; position: absolute; top: 100px; height: 100px;">
<asp:contentplaceholder id="ContentPlaceHolder1" runat="server">
</asp:contentplaceholder>
</div>