You get measured when you join and over the next few weeks/months you argue with the suppliers about what they have issued. Normal gripes are are trousers too long/short, wrong number of shirts, long sleeves instead of short (or vice versa), incorrect epaulettes etc. Your uniform is normally dispatched to the wrong base with the wrong name and you'll be lucky to get a correct and complete issue before your line training starts. For pilots (fortunately) there is no wings ceremony, they are just nailed in your jacket when you join.
PM