it can be done
if file is fixed block
length is =5+length of alternate key+n*4 bytes(rba)
else if file is variable block
avg length is =5+length of alternate key+n*4 bytes(rba)
n is avg occurence of duplicates in alternate key
max length is =5+length of alternate key+n*4 bytes(rba)
n is maximum occurence of duplicates in alternate key