This is entirely up to you - for this recipe, I sliced cremini and shiitake mushrooms in fourths to give a classic sliced mushroom shape - but the other mushrooms I add in that don't have a uniform texture I just roughly chop into bits about the same size as the cremini slices. The shape doesn't matter as much as trying to have bits about the same size, so they all cook uniformly. Mushrooms with very small caps, like enoki, tend to just be joined at a woody, dirty base - I will just cut off the base and use the small caps with some of their long stem attached. Be sure to remove any woody, tough bits from the base of mushrooms, as they add nothing to the flavor and will be tough (and unpleasant) to eat.