How can I set the PowerShell default window size in Windows 7?

Go Start -> All Programs -> Accessories -> Windows PowerShell. RClick on the one you're using. Go to the Layout tab, and change the settings you want.

Use a function like this in your profile (notepad $profile)

Function Set-WindowSize {
Param([int]$x=$host.ui.rawui.windowsize.width,
      [int]$y=$host.ui.rawui.windowsize.heigth)

    $size=New-Object System.Management.Automation.Host.Size($x,$y)
    $host.ui.rawui.WindowSize=$size   
}

Then call the function:

Set-WindowSize 100 50

Another approach is to run Powershell as an administrator. See How do I modify the Powershell console settings in Windows 7?