size-of-installed-games.ps1

function Get-Games {
    # Game installation locations
    $gameDirectoryNames = @("Genshin Impact", "Diablo II Resurrected")

    return (
        (Get-ChildItem "C:\Program Files (x86)\Steam\steamapps\common") +
        (Get-ChildItem "C:\Program Files (x86)\GOG Galaxy\Games") +
        (Get-ChildItem "C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\games") +
        (Get-ChildItem "C:\Program Files" | Where-Object { $gameDirectoryNames.Contains($_.Name) }) +
        (Get-ChildItem "C:\Program Files (x86)" | Where-Object {  $gameDirectoryNames.Contains($_.Name) })
    )
}

function Format-Size {
    param($Size)

    if ($Size -lt 1GB) {
        $value = [math]::Truncate($Size / 1MB)
        return "$value MB"
    } else {
        $value = [math]::Truncate($Size / 1GB)
        return "$value GB"
    }
}

# returns content size of a directory in bytes
function Get-Size {
    param($FullPath)

    return (Get-ChildItem -Recurse -File -Path $FullPath | Measure-Object -Sum -Property Length).Sum
}


$items = @()
Get-Games | ForEach-Object { 
    $sizeInBytes = Get-Size -FullPath $_.FullName
    $size = Format-Size -Size $sizeInBytes
    $name = $_.Name
    $path = $_.FullName

    $items += @{
        "Name" = $name;
        "SizeInBytes" = $sizeInBytes;
        "Size" = $size;
        "Path" = $path
    }
}

$items | ForEach-Object { [PSCustomObject]$_ } | Sort-Object -Property "SizeInBytes" -Desc | Format-Table -Property Name,Size

Output:

Name                                          Size  
----                                          ----
ForzaHorizon5                                 102 GB
Horizon Zero Dawn                             72 GB
GodOfWar                                      64 GB
Cyberpunk 2077                                61 GB