Hey Carmeow
I can imagine how frustrating it must be to get an outfit only to discover that it just doesn't fit. Here is a list of measurements to take before buying your new outfit or spending so much time making one.
Neck - Measure around her neck like you would for a collar. Add a little room to make sure its comfortable.
Chest - Measure her chest at its widest point, all the way around, which is usually right behind the front legs.
Length - Measure along her backbone from the base of her neck to the base of her tail. This is where her collar rests to where her tail meets her bum I hope this helps and I'm sorry to hear that your upset 
Next time it will be perfect!