As has been described in a few reply’s already, to make a black hole you have to “squeeze” an amount of mass to a size so small the density will be enough to create a black hole. This size is given by a radius called the Schwarzchild radius. The equation is R = 2GM / c^2 where G is the gravitational constant and c is the speed of light. M is the mass in question. c^2 is a huge number compared the the numbers it is dividing meaning you need a LOT!!!! Of mass (M) for R to be a meaningful to a human scale measurement. As a slight aside, Saying something like shrinking the sun to a sphere 5km across is a little meaningless to most of us.. we understand 5km but we absolutely cannot comprehend the sheer size of our own sun. Back to tiny black holes.. the other part you need to understand is that physics has shown that there is an fundamental smallest distance that still makes sense… that distance is called the Planck length and is about 10^-35m or a decimal with 35 0s and a 1 on the end. So you can see that if the mass is relatively small it will not be able To make a black hole because it would need To be squeezed in to a size smaller than the Planck length.