First off, I'm sorry for your loss.
Your 1100 will shoot the 2.5" shells just fine. As far as self defense, the lower the number on the ammo, the larger the shot. So 8's are small and 00 buckshot is larger.
I'm not very familiar with 20 gauge, so I can't really recommend a good SD load.
“If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that ye were our countrymen.” ― Samuel Adams