IEEE Std 802.11-2016, 9.4.2.2
The maximum length of an SSID is 32 bytes, even when the SSID should be
interpreted using UTF-8 encoding (see Table 9-135). While every length in
the 802.11 standard is byte oriented, the word BYTE is included in the
name of this constant to emphasize the importance that it not be applied
to the length of a UTF-8 encoded string.
The limit on the number of channels in a list of unique channel numbers is 256
since a channel number in IEEE 802.11-2016 cannot exceed one octet. See
IEEE 802.11-2016 9.4.2.18 Supported Channels element for an example element
that assumes a channel number does not exceed one octet.
The limit on the number of SSIDs is 84 because an SSID List IE can contain no
more than 84 one byte SSIDs. (Specifying a zero byte SSID in an SSID List with
more than one SSID is valid but unnecessary since it is the wildcard SSID.)
IEEE Std 802.11-2016, 9.4.2.25.1
IEEE mentions that an element body maximum length is 255 octets in the RSN element
section, but not in a dedicated section.
Since the IE header is two octets, the whole IE max length is 257 octets.