Powershell folder size of folders without listing Subdirectories

This simple solution worked for me as well.

powershell -c "Get-ChildItem -Recurse 'directory_path' | Measure-Object -Property Length -Sum"

The solution posted by @Linga: "Get-ChildItem -Recurse 'directory_path' | Measure-Object -Property Length -Sum" is nice and short. However, it only computes the size of 'directory_path', without sub-directories.
Here is a simple solution for listing all sub-directory sizes. With a little pretty-printing added.
(Note: use the -File option to avoid errors for empty sub-directories)

foreach ($d in gci -Directory -Force) {
  '{0,15:N0}' -f ((gci $d -File -Recurse -Force | measure length -sum).sum) + "`t`t$d" 
}

You need to get the total contents size of each directory recursively to output. Also, you need to specify that the contents you're grabbing to measure are not directories, or you risk errors (as directories do not have a Length parameter).

Here's your script modified for the output you're looking for:

$colItems = Get-ChildItem $startFolder | Where-Object {$_.PSIsContainer -eq $true} | Sort-Object
foreach ($i in $colItems)
{
    $subFolderItems = Get-ChildItem $i.FullName -recurse -force | Where-Object {$_.PSIsContainer -eq $false} | Measure-Object -property Length -sum | Select-Object Sum
    $i.FullName + " -- " + "{0:N2}" -f ($subFolderItems.sum / 1MB) + " MB"
}