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

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

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.

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