Its fine if you use a guard with a mandoline - which I always do.
I use a mandoline all the time. I typically slice things I'm pickling to 3 mm, but sometimes 2 mm. Along those lines, I find myself cutting colossal diakon radishes into quarters before slicing so it works better for kimchi (and so it fits better into the jar).
I generally hold the vegetable in my hand until it gets closer to the nub, then I use the guard.
To the original question, I want whatever I'm slicing/dicing/chopping to be reasonably edible in a single bite. As SatNavSaysStraightOn notes, that can be a different size for different people. But, if I'm using chopsticks, I don't like having to pick up a massive chunk that I have to take a bite out of and put down. It's a similar story with a fork, except that there's normally a knife nearby that I can use to cut down those chunks.
I haven't tried measuring the size of the chunks, but I'd say that tablespoon-size would be the maximum I'd like to use. Here, I'm talking about dishes with the sliced/chopped/cut thing incorporated (like a stir fry). The exception would be if I have shrimp/prawns in a dish. I don't want to pre-cut those for convenience, since I like the visual of the whole item.