There are several ways:
Write-Host: Write directly to the console, not included in function/cmdlet output. Allows foreground and background colour to be set.
Write-Debug: Write directly to the console, if $DebugPreference set to Continue or Stop.
Write-Verbose: Write directly to the console, if $VerbosePreference set to Continue or Stop.
The latter is intended for extra optional information, Write-Debug for debugging (so would seem to fit in this case).
Additional: In PSH2 (at least) scripts using cmdlet binding will automatically get the -Verbose and -Debug switch parameters, locally enabling Write-Verbose and Write-Debug (i.e. overriding the preference variables) as compiled cmdlets and providers do.
Powershell has an alias mapping echo to Write-Output, so you can use:
echo “filesizecounter : $filesizecounter”