Here is an alternate solution that may work for you.
Save this following code in ScriptEditor.app, as an application. Then in Finder.app, while holding down the command key, drag that script editor application you just saved, to the Finder's toolbar. Now anytime you have a file selected in Finder, click that .app you just moved to the toolbar, and it will display a dialog with the file size of the currently selected file.
tell application "Finder"
try
set fileSize to size of item 1 of (get selection)
on error errMsg number errNum
activate
display alert ¬
"Selected File Info" message ¬
"There Is No Selected File In Finder" buttons {"OK"} ¬
default button ¬
"OK" giving up after 4
return
end try
end tell
copy fileSize as string to fileSize2
set tempList to text items of fileSize2 as string
set tempList to tempList as inches as text -- Workaround For Displaying Large Numbers Correctly
try
set kiloBytes to ((items -4 thru 1) of tempList & "." & (items -1 thru -3) of tempList)
set theMessage to "The File Size Is " & kiloBytes & " Kilobytes"
on error errMsg number errNum
set theMessage to "The File Size Is " & fileSize & " Bytes"
end try
activate
display alert ¬
"Selected File Info" message theMessage ¬
buttons {"OK"} ¬
default button ¬
"OK" giving up after 4