Are you starting in 1090? If not, just expand a lot from 1066. If you can field as much, or just a few less as the HRE, your team will have few problems, depending on how much support you get from the rest of Europe. The Pope usually has large stacks, but Egypt will have bigger ones. Defend his stacks with yours, but sometimes you might want to wait till the Egyptians get to a river crossing. Usually something like a 5/8 ratio will win on the defense at a river crossing. Sometimes its a lot better, and you can win with half or a little more.
Reinforce again once your troops die. Keep slinging men at the enemy. I hardly ever have a failed first crusade (I hardly ever see the AI fail their Crusades, even). The biggest thing to make sure of is that your King is NOT commanding in the front lines. If he dies, you will be out of the crusade.