There’s some variation for size, but when I’m doing it as transport, close the hand around her; otherwise it feels like she might fall off. At the same time though, when it’s not about moving her, or at least not moving her any real distance (to me, anyways), an open hand lets me look at her better, and for her to get a better look at me in turn, while also allowing her to feel less trapped
If they’re small enough, a couple of inches or so, pinching between fingers becomes more tempting as the ‘display’ hold, though if anything I’d be even less likely to try and transport her that way if she’s so small and easy to lose track off; that’s the point where just closing her up in my hand becomes tempting.
Early on in the relationship (unless I’m playing hard ball for some reason), I’d want her to willingly come to me and walk into my hand whenever possible, to build trust so that she’s not too afraid, but as time passes and we settle into our relationship, and she gets used to be picked up, I’d probably be more and more casual about just… grabbing her, often with little to no warning.