When you have more than one portlets on a page and you want to set size of portlets, you just make use of Show Layout tools.
If it is disabled, just enable it and set size of portlets in either pixel of number.
Here you go....
Enable Show Layout Tool link
1.Login to portal as admin and click on Manage pages.
2.Navigate to you page where you have added portlets.
3.Click on Edit Page Layout of the page
4.Go to Edit_shared_setting of Edit Layout portlet
5.Select the check box for Show toggle link for "Show layout tools/hide layout tools"
You should see Show layout tools link.
Setting portlet size
1.Click on Show Layout tools link
2.You should now see Not Set link in each container
3.Click on Not Set link and there you have pop up, where you can specify portlet width in pixel or a number followed by "%" Example: 30% or 200.