FIR filters always linear?
Yes, FIR filters are linear.  No, the length of the impulse response does not have anything to do with whether it is linear or not.  For instance, IIR filters are also linear.

Linearity is determined by whether the filter obeys the superposition principle, nothing else.  So, FIR and IIR filters are linear, median filters are not.

EDIT: Technically Dilip is correct.  Finite and infinite impulse response filters can be non-linear if their impulse responses incorporate non-linear operations, such as squaring.  I was going by the "industry" meaning of FIR and IIR, which are linear because they only use multiplication and adding, but in the mathematical sense FIR and IIR filters can be non-linear.