First few weeks

[Joygrrr]

My LO is only a few weeks off starting to wean, I've tried him with a spoon of apple purée to gauge his reaction and he loved it but as he's only 21 weeks I want to hold off a few more - my question is once he does start to wean what portion sizes consitute a 'meal' ? I know they don't replace feeds in the early weeks but once they do what are we talking, 4 spoons of purée? Or a whole bowl? Or just what ever LO wants to eat? Also is ,say apple purée, a meal? Or do I need more with it for it to be a meal, like porridge or baby rice
Advice please!