it depends on who makes the DVD, when the DVD is made, and when you obtained your DVD. Alien Bob's mirror-slackware-current.sh appears to include the /patches directory on the install CD. So the size of the disk may vary based on time and how the install CD was generated.
You can however use the CHECKSUMS.md5 to check the sums of each file individually... If it boots and all the checksums are fine, you should be good to go.
md5sum -c CHECKSUMS.md5
This will output a lot of lines... if you end up with an error or a warning at the very end, you may need to send the output of the above command to a text file "> /path/to/log.txt"