How big is the universe?

That is not currently known. The observable Universe has a radius of about 46 billion light-years. The entire Universe is probably much larger.
The universe is so, extremely giant, it's pretty much impossible to actually imagine how big it really is. The best thing to do is to start with something small and work up from there; so let's start with the Earth. Earth may seem a giant place to us humans, but no where near in size to the universe. But, at approximate, the Earth is about 8,000 miles wide. If you drove a transglobal tunnel down through the middle of the Earth (which is not possible, but just imagine it) at a non - stop speed of 60mph, then you'd pop out the other side in about 5.5 days. Which isn't that long with what will come up. Now, lets try the distance to the Earth to our Moon. Every month, the Moon will get further and closer away from us. But, on average, the distance from Earth to Moon is 240,000 miles. Travelling at a speed of 60mph, you'd get to the Moon in about 168 days, which is quite long. Now, lets try something even longer; the Earth to the Sun. This distance is about 92 million miles, and would take you about 176 years to get there by 60mph. Now, to cross our galaxy, the Milky Way. By going at 60mph, you'd have to travel 621 million billion miles to cross the Milky Way. It would take 1 million billion (or, to be more precise, 1,181,401,000,000,000) years to accomplish. So, this tells us that a space car would be pretty cool, but 60mph is a pathetic speed for travelling across space. So, we'll think of something super - duper fast; light. The speed of light can travel a mind - blowing 186,000 miles in 1 second, at approximate. We're talking about a speed so fast it could travel 6 trillion miles if it kept on moving at it's normal speed for 1 year. This is known as a 'Light year'. But, even with our super fast speed of light travel, it'd take it about 100,000 years at approximate to cross the Milky Way. And, to move onto our universe. With out most powerful telescopes, we can measure that the universe is 15 billion Light years in every direction. So, at the speed of light, it'd still take at least 30 billion years to cross the universe.

For a conclusion, it's pretty much impossible to measure out the proper size of the universe.
It never ever ends. It goes on and on and on ........
It is theorized to be infinite but there is no way to know with our technology because we can only see a certain distance using different types of technology.