I had heard of the same myth when I started out. I now feed in the enclosures, and can say they have a stronger feeding response, and no aggression. As long as you open the enclosure to change water and clean, you should have no problems.

Usually slightly smaller than the widest part is better. Sometimes they'll balk at the size alone. Anyway that's my experience on the subject.

Good luck on your first feed, and let us know how it goes.