It would seem to me that the assignment is for two and a half pages because you are expected to devote a half-page to explaining briefly who these two characters are and what they do in the respective works. Don't take it for granted that the reader already knows who they are and what happens to them. The paper should be written as if addressed to the general reader and not to the teacher. Then you are probably expected to devote one page to comparing them and one page to contrasting them. I believe you should do both.
Snowball and Ralph are both leaders. They are both well-meaning types, thinking of the general welfare. Both are overthrown by individuals who are inferior to them and selfishly motivated. Both are in danger of losing their lives. Each has one deadly enemy.
They are different in one obvious way. Snowball is a pig and Ralph is a human being. Thus it would seem that Ralph must be much more intelligent than Snowball. Snowball is Utopian. Ralph is just trying to keep everybody alive and civilized. Snowball is an ideologist. Ralph is a realist. Snowball represents a real-life person, Trotsky. Ralph is not a character in a fable, but in a realistic sci-fi novel. You can appreciate Lord of the Flies without referring to anything else,whereas you need to know something about the Russian Revolution and the subsequent history of the Soviet Union in order to appreciate Animal Farm. I think Animal Farm might be called a roman a clef as well as a fable, an allegory, and a dystopian novel. Lord of the Flies is a sci-fi novel with a message.
You could make some brief generalizations about the two works in your conclusion.
These are just rough ideas off the top of my head. If you refer to the copious information available on both these works in eNotes Study Guides, I'm sure you will get more ideas about how these two characters are alike and different.
When you're comparing, just compare. When you're contrasting, just contrast. Don't get involved in doing both at once. It's too confusing.