Show human-readable file sizes in the default PowerShell ls command

try this

PS> gc c:\scripts\type\shrf.ps1xml

<Types>
<Type>
  <Name>System.IO.FileInfo</Name>
   <Members>
      <ScriptProperty>
          <Name>FileSize</Name>
          <GetScriptBlock>
             switch($this.length) {
               { $_ -gt 1tb } 
                      { "{0:n2} TB" -f ($_ / 1tb) }
               { $_ -gt 1gb } 
                      { "{0:n2} GB" -f ($_ / 1gb) }
               { $_ -gt 1mb } 
                      { "{0:n2} MB " -f ($_ / 1mb) }
               { $_ -gt 1kb } 
                      { "{0:n2} KB " -f ($_ / 1Kb) }
               default  
                      { "{0} B " -f $_} 
             }      
          </GetScriptBlock>
     </ScriptProperty>   
  </Members>
</Type>
</Types>

PS> Update-TypeData -AppendPath c:\scripts\type\shrf.ps1xml -verbose
PS> get-childItem $env:windir  | select Name,FileSize,length
PS> # you can paste this in your profile
PS> 

you can also use dynamic type data with PS3:

   PS> Update-TypeData -TypeName System.IO.FileInfo -MemberName FileSize -MemberType ScriptProperty -Value { 

    switch($this.length) {
               { $_ -gt 1tb } 
                      { "{0:n2} TB" -f ($_ / 1tb) }
               { $_ -gt 1gb } 
                      { "{0:n2} GB" -f ($_ / 1gb) }
               { $_ -gt 1mb } 
                      { "{0:n2} MB " -f ($_ / 1mb) }
               { $_ -gt 1kb } 
                      { "{0:n2} KB " -f ($_ / 1Kb) }
               default  
                      { "{0} B " -f $_} 
             }      

 } -DefaultDisplayPropertySet Mode,LastWriteTime,FileSize,Name

First, create the following function:

Function Format-FileSize() {
    Param ([int64]$size)
    If     ($size -gt 1TB) {[string]::Format("{0:0.00} TB", $size / 1TB)}
    ElseIf ($size -gt 1GB) {[string]::Format("{0:0.00} GB", $size / 1GB)}
    ElseIf ($size -gt 1MB) {[string]::Format("{0:0.00} MB", $size / 1MB)}
    ElseIf ($size -gt 1KB) {[string]::Format("{0:0.00} kB", $size / 1KB)}
    ElseIf ($size -gt 0)   {[string]::Format("{0:0.00} B", $size)}
    Else                   {""}
}

You can then pipe the output of Get-ChildItem through Select-Object and use a calculated property to format the filesize:

Get-ChildItem | Select-Object Name, @{Name="Size";Expression={Format-FileSize($_.Length)}}

The function could of course be improved to account for sizes in the PB range and more, or to vary the number of decimal points as necessary.