1. Size is not always a good characteristic to use for identification. As with humans, butterflies vary in size, so although most Large White individuals are bigger than Small Whites, they can occasionally be smaller. Black/grey spots on the wings are also not very helpful, as these vary between males and females as well as between species.
2. You will have to see the underside of the hindwing to be certain of the identification. This means that if you only see the butterfly fly past at a distance, it is very unlikely that you’ll be able to make a definite identification. You have to be patient and accept that it won’t be possible to identify every white butterfly that you encounter!