Based on various previous editions of RQ as well as MRQ2 my rule has always been (and is certainly embedded in Age of Treason) that 21 is the Max human limit - so Characteristics have a dice roll range of 3-18 or, for INT and SIZ, 8-18, and can be trained/enhanced up to 21. If playing for a touch more realism, limit the increase also by + 33% or +50% of original rolled.
If a characteristic is forced below minimum (ie to less than 3 or less than 8 for SIZ and INT) the character is disabled. If it hits 0 he is dead.
The same logic applies to non-humans, so maximum is Max rollable plus number of dice, treating each +6 or fraction thereof in fixed addition as one dice (Hence a creature with a characteristic of 2D6+12 has a species max of 28).
Human Characteristics can be increased above 21 temporarily through magical enhancement - but can only achieve this only permanently through an actual change in the individual that makes them more than human (this is a fundamental in AoT setting) - ie becoming some sort of hero or god. If you were running a Heroic Greek campaign, you would, under this system, allow heroes one or more stats in excess of 21 to reflect their status as demigods.