A number of issues are just appearing to me.
Asking for help is a sign of hope that you wish to learn more of your system. Bribery says, you don't care about putting time into the thing. First is cool, second is...
There is major changes between each of 5 editions(6, if you count 3e and 3.5 as two separate entities), Which one?
Given the fact that you use the term lawful neutral, makes me guess 3.x.
If it is 3.x, 3 inches is fine, not tiny. In short; WAY TOO SMALL!!!
Robin Hood fought the a tyrannical ruling class. That makes him Chaotic or the Neutral. The fact that he wouldn't work with the law, would put him closer to Chaotic. Chaotic Good to be more precise.
Many DM's might argue that using poisons would be an act of evil in of it self.
Ether way, I would suggest looking at a Anthomorphic Animal(Rat) in Savage Species for the Race base.