There is an effect called: Sharpen

There are 2 options - Amount (0-1000) and Size (0-1000).

Could you explain it how to use it properly and what each of those do?

Is there any preview option like in a GIMP?

This is a port of the Mplayer's "Unsharp mask" code, so the parameters determine the blur radius of the unsharp mask ("Size") and the amplitude of the difference signal ("Amount"), as usual in unsharp masking.

The image in the project monitor should change immediately after each parameter change, no need for special preview mode.