WL#3310: Update ndb_size.pl for 5.1

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

ndb_size.pl is proving popular.

It's calculations for 5.1 are not 100% accurate.

Need to check ndb_size.pl against current documentation and any knowledge
sitting in the heads of developers.

Also some real-world testing to verify it is actually correct.