The layout of a portal page is determined by the theme and skin in which it resides.
Tables and style sheets can be used to manipulate the layout within a portlet, but you can't control the overall size. All you can do is develop a portlet that will look good in any environment.
Having said that, if your portlet is given 400x400 pixels, there's no reason you can't just put the whole thing in a table 200x200 in size. There'll just be
alot of whitespace, that's all.
-Cameron McKenzie