From IEEE 802.11-1999 22.214.171.124.3.2.1 Service Set Identity (SSID) element
The SSID element indicates the identity of an ESS or IBSS. See Figure 35.
The length of the SSID information field is between 0 and 32 octets. A 0 length information field indicates the broadcast SSID.
That's a bit misleading as the SSID is sometimes null terminated leaving only 31 characters available. Some firmware versions screw up if you use the full 32 characters.
Spaces are just fine. Some utilities will blow up if you do that, but most access points handle it just fine.
Maximum humor, keep it clean, forget about bell characters, and no nulls.