Event horizons are regions around gravitational singularities withjin which light signals can not escape into the outside universe.

In general relativity, they appear as coordinate singularities where for non-rotating black holes for example, the Doppler factor increases to infinity so that 'infinite redshifts' and 'infinite time dilation' occurs there. If, however, you choose a coordinate system freely falling into the black hole, no singularity occurs at the event horizon radius, and you can enter the black hole, but only after an infinite time as observed bu distant observers. For you, the trip may last only seconds assuming you survive the gravitational tidal effects!

If you want to calculate the size of a horizon for a given mass for the black hole, there is a neat Calculator you can use at the Event Horizon page of 'Into the Cosmos' The rule of thumb is that for every increment of solar mass in the black hole, its radius increases by 2.7 kilometers if it is a non-rotating black hole. The Earth as a black hole would be about one centimeter across.

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