How big is a paragraph?
There's no fixed number of sentences that make up a paragraph, just as there's no fixed length for a sentence. Generally, though, aim for a happy medium in the papers you write for school. One-sentence paragraphs, which we often see in newspapers, fail in many other situations because they don't provide enough context: they jump in from out of the blue, then leave before we can figure out what they're talking about. However, in some cases, they can be effective, if used sparingly. Extremely long paragraphs, on the other hand, can overwhelm readers, because we need breaks where we can process information before moving on.
So, then, where do you break your paragraphs? That is a tough one. Each paragraph theoretically covers one subtopic, so the paragraph should be as long as the subtopic requires. Of course, you also need to conceive of subtopics that are paragraph-size. It's something of a paradox, and getting a feel for it takes lots of practice. But you can postpone most paragraphing decisions until your second draft, when you'll have a better sense of what your subtopics are.