$columns = ($datatable | Get-Member -MemberType Property).Name
foreach($column in $Columns) {
$max = 0
foreach ($a in $datatable){
       if($max -lt $a.$column.length){
        $max = $a.$column.length
       }
}
$output = New-Object psobject
$output | Add-Member NoteProperty Column ($column)
$output | Add-Member NoteProperty Max ($max)

Write-Output $output
}

Column Max
A       3
A       4
A       5
B      10
C      19
C      29
D       9