Echo 64 bytes to a seed file:
C:\Temp>echo This is just a sample line appended to create a bi wefhwef weh> dummy8.txt
Then call 'type' in a for loop to append the contents of the file to itself:
C:\Temp>for /L %i in (1,1,8) do type dummy.txt >> dummy.txt
Makes a 32,768 byte (32Kb) file.