I think this is the most difficult question I’ve seen during the contest!
The honest answer is that we don’t know for sure, but we think it’s about 93 billion light years in diameter. This is based on the age of the oldest stars we have been able to detect: 13.8 billion years old! That means that the light we are seeing know was emitted by the start 13.8 billion years ago! But the universe has been expanding since, and we know how much: those old starts are now 46.5 billion light years away. Doubling that up we get the diameter, 93 billion light years.
PS: Distances are so big when talking about the universe that we use light years instead of kilometers. A light year = 9,500,000,000,000 kilometers, or in other words the distance that you would cover when travel at light’s speed for a year.