You can use AutoSizer to keep your windows at a specific size. It's not the perfect solution, but I'm not aware of any other way. From the website:
AutoSizer automatically resizes
programs for you, keeping them at a
specific size or keeping them
maximized. One popular use is to keep
Internet Explorer maximized when you
go on the web. It works with most
software programs, and supports
maximizing to specific monitors on
multi-monitor computers. Use the handy
Always On Top system tray menu to
quickly position specific programs
above all others.
If that doesn't work you could also try out Sizer. It allows you to resize a window to one of a few predefined resolutions.
To save your icon positions, you can use DesktopOK . From Lifehacker:
Tiny application DesktopOK saves and
restores your desktop icon
positions—so your perfect icon layout
won't get lost if your resolution
changes when hooking to another
monitor or playing a game.