As defined in nbase.h (or in some system header file on Linux), MAXHOSTNAMELEN is 64. This is the maximum length of a hostname, but not of a DNS name, which may be longer. RFC 1035 spells out that DNS names are formed of labels, each of which must be 63 octets or less, and that labels are combined to form a DNS name (a.k.a. FQDN) which may not exceed 255 octets. Each label is length-prefixed, so the ASCII readable name may only be 253 characters.

Nmap currently uses MAXHOSTNAMELEN in some places where the 253-character length should be used instead. As a simple example (because nmap.org has wildcard DNS), you can scan the 71-character DNS name "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789.nmap.org" with a short timeout to see that the message about "Skipping host due to host timeout" truncates the name at 64 characters:

$ nmap -p 80 abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789.nmap.org --host-timeout 1ms -Pn

Starting Nmap 6.40 ( http://nmap.org ) at 2015-05-28 18:32 UTC
Nmap scan report for abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789.nmap.org (173.255.243.189)
Host is up (0.039s latency).
rDNS record for 173.255.243.189: nmap.org
Skipping host abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789.n due to host timeout
Nmap done: 1 IP address (1 host up) scanned in 0.08 seconds

Nmap also uses this length for the proxy host for Idle scan (-sI) and for the FTP server name for Bounce scan (-b). Nping uses MAXHOSTNAMELEN, but defines it to be 128. A more complete audit should probably be made.