how do we know the universe is expanding and not us shrinking?

Not just the Earth, but all matter. There would be no way to tell the difference. If my ruler is shortened by half, wavelength would double.

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Richard dawkins once told the story of two individuals.

One individual says that its interesting how the earth/planets actually rotate around the sun. The other responds, yeah I know, it looks as though the sun orbits around the earth. The first individual responds, so how exactly would it look if the earth orbited the sun?

And evidence for the expansion lies in the cosmic microwave background, and in red shift calculations etc.

In response to what you said, before edwin hubble first made his observations on the galaxies, he expected to see a similar number of blue shifted spectra as red shifted ones. When he actually did the observations, most galaxies appeared red-shifted, meaning they were moving away from us.

Also perhaps you should look into the holographic principle--

Every so often, the distance between our planet and other galaxies, stars, etc. is measured. Scientists have found out that they appear to be growing FURTHER away. Stars are becoming dimmer, galaxies are becoming dimmer and smaller, etc. And it's the same way on each side of us because we can only see as far as our horizon allows us. It's becoming spaced out. It's one way of showing that the universe is actually expanding. And it's speeding up as the years go by, contrary to what was originally believed.

By measuring the red shift of distant galaxies,we made conclusion about expanding universe.

So far all research shows that it is not only expanding but accelerating its expansion.

Doppler effect. Stick a brass band on a flatbed ttrailer and have a driver drive it past you, the sound gets louder as it approaches you and diminishes as it passes you and moves away.

Light does the same way, blue light moves faster and red light moves slower. We can tell the difference between a sunrise picture and a sunset picture by the blue or red light.

Please tell me I don't have to connect the dots.

There was a test done in 2011 proving this but i dint remember it well. It did win the nobel prize in physics

Because the galaxies and everything in the universe is getting further and further apart. That wouldn't happen if it were shrinking.

Read any science textbook and get your answer.