Smallswords are designed for one thing, and that's speed.
The weapon Brienne is practicing with is built very differently.
Even if Arya had the beefiest arm in the world and was much stronger than Brienne (and come on, is there anyone in Westeros stronger than Brienne?), Needle still wouldn't have been strong enough to block the force of her sword.
Now, I still believe that Arya could've won in a fight against Brienne.
I think Arya is fast enough that she could've water-danced circles around Brienne and tagged her before Brienne could've landed a blow.
She even proved that earlier in the episode when she dodged the guards at the gate.
But being able to block Brienne with a smile on her face? I'm sorry, but...not today.