How long should a eulogy be?
Ask the person coordinating the service how much time you have. If nobody has set a limit, a concise eulogy is usually easier to deliver well. Practice aloud and remove anything that only makes sense because you know the backstory.
What if I am afraid I will cry?
Print the words in large type, mark places where you can pause, and give a copy to someone who could continue if you cannot. Becoming emotional does not mean you have failed to speak well.