You just need to 'play it by ear' really - also it depends on their likes and dislikes - and their age. Give them so much, see if they eat it all and offer more if appropriate.... it's ok saying 'half fill a bowl' (no offence meant) but how big is the bowl???? For older children they will usually indicate how much they want (and if teenagers they definitely will usually by means of sarcasm....ie 'is that all i get?'!!!!) Rice and pasta are quite filling too.