Here is what I did, works a charm.

You will need to make a bat file, in your documents or where ever you want to save it.
then right click it and edit
then copy and paste this, with out quotes

"@echo off
start Yahoo! UK"

then save it (this is one for yahoo uk) just replace it with your own url.
then right click and create shortcut.
then move the shortcut to your desk top.
then right click the shortcut , go to properties and under the shortcut tab, go to the run box and change to minimized, yes that's right "minimized. you can change your icon there as well.
then close it.
once done your window will open maximized every time.