What would happen if we take all matter/energy in the visible universe and turn it into a black hole

What would be the result?

How large would the event horizon be, and what would be the density inside such a black hole?

it would have infinite density...

Well firstly, the larger a black hole is, the less its density becomes, as density is dictated by the schwarchild-radius's distance from the initial event horizon. The biggest black holes we know of at the centre of galaxies have densities comaprable to water. Secondly, such a black hole produce enough gravity to devour whole galaxies into its ether.

then youd just have a black hole since you took it and made it into a black hole...