How do scientists know how large the universe is?

Scientists work out how large the universe is by paying attention to each little measure meant eg.

It's surprising at how far away the moon is from Earth (384 400 km away to be exact). The Oort Cloud contains our solar system and even one of the space-explorers will take 30 000 years to reach out of it. Once scientists move out of the Oort Cloud and into our stella section with our stellar neighbours, they use a measurement called light years to measure from there.

After that, they zoom into the Milky Way. And they measured the Milky Way to be a staggering 100 000 light years long from end to end! Then they zoom out into the local group of 54 galaxies which is about 10 MILLION light-years across!

Then finally it is the Virgo SuperCluster where there are 100 more groups of galaxies. This group measures across 110 MILLION light-years. And this one if only tiny in another group called the Laniakea SuperCluster that is 520 million light-years long! And even more mind-numbing, this is only a speck of the entire universe!

SIMPLER SUMMARY:

They basically measure our solar system and multiply it by how many other solar systems they know of and this is a repeated process. They still don't know how big the galaxy is because it contains over 2,000,000,000,000 singluar galaxies.

Pretty fascinating right?

Hope this helped you out understanding how they know.
This video is extremely helpful to understand:

Hope this helped you out!