Although goggles vary slightly in shape and size, they do not have a sizing break (S,M,L,XL). There are only 2 sizes to consider, and to decide what is the right goggle for you all you need to do is look at what helmet you wear.
All kids (youth) sized helmets will only work with a kids (youth) sized goggle. An adults goggle will not fit inside the eye port, at best it may sit on the outside of the helmet but the foam will not come in contact with the riders face allowing dirt and dust to work its way into the riders eyes.
If you have an adults size helmet and have a set of kids goggles you will probably struggle to get the strap over the helmet for starters. On top of this the foam will end up touching your eyes as the frame will be far too small for your face. Remember you want a goggle where the foam sits evenly on your face and creates a seal to prevent dirt from seeping into your eyes!