Actually the MISRA:2004 Rule 5.1 for C90 with a maximum of 31 characters is supported by the SonarCFamily-Plugin of the developer version.
In MISRA:2012 four rules (5.1, 5.2, 5.4 and 5.5) are used and they differ between internal and external identifiers and the usage of C90 or C99. So the maximum characters allowed differ between 31 and 63 characters.
Rule 5.1: 31 characters for external identifers with C90 and C99
Rule 5.2: Identifiers in the same scope and name space-> 31 characters with C90 and 63 with C99
Rule 5.4: Macro identifiers-> 31 characters with C90 and 63 with C99
Rule 5.5: Identifier shall be distinct from macro names-> 31 characters with C90 and 63 with C99