My parrotlet loves her fruit and veg chopped smaller, but loves her kiwi and broccoli big and chunky and won't entertain it if it's chopped small.
My grey loves everything chopped at a size where he can hold it. Then he will only eat mango if it is cut long and chunky, if you cut it any other way you have ruined his morning
My parrotlet loves fruit and veg cut any way or grated but i have found if my grey stops eating a certain veg if i cut it different it brings his attention back to it. As DianaT says, trial and error
Different birds Different likes, its all down to trial and error. if I was to give some birds the whole fruit or veg it makes them work more for there breakfast I often hang the whole fruit/veg up on rope but I cannot say it dose not make more mess but how my flock love it