Write files to folders according to file size

It is possible, you have to just check the file size first and after that you can separate them with if case by size as you want.

Check the follows:
How to: Determine a File's Size in Visual Basic[^]

Dim infoReader As System.IO.FileInfo
infoReader = My.Computer.FileSystem.GetFileInfo("C:\testfile.txt")
MsgBox("File is " & infoReader.Length & " bytes.")

You can use this as the follows:

Dim infoReader As System.IO.FileInfo

infoReader = My.Computer.FileSystem.GetFileInfo("C:\testfile.txt")

If infoReader.Length < 1000 Then
    'your code for separation

Else
    'your code for equal or bigger than 1000 bytes file

End If