So, eliminating "All these physical requirements", we have:
Stanley: "...
"Now think. I want a guy who's commanded all different kinds of battles.
"I want him to be obsessed with war. Somebody who plans wars and kills his foes for fun.
"I want a guy who snacks on gwiffons and eats Marbits for BREAKFAST!
"... Get me somebody who actually wants to be summoned."
Wanda: "Yes Lord."
Stanley: "...
"Everything should seem familiar and safe to him. No crazy surprises."
Wanda: "..."
Stanley: "Oh all right, find your genius. Just make him BIG, okay?
"Can you do that much, Commander?"
And we have a description of Lord Hamster.