A size smaller does not mean the same thing as a smaller size. A size smaller would be from size 8 to size 7, for example, that is, ONE size smaller. A smaller size could mean any size smaller than the size in question.
Because it matches размер. With нужен, the object needed is in the nominative and the person who needs is in the dative case.
I put that....but then thought it means one size smaller whereas a smaller size could mean several sizes smaller
That is proper English used commonly in High Streets up and down the country here in the UK. Report it.
What exactly does "EI nuzhen" mean in Russian? Is it "Her need" or "She needs"?
So, I kinda remember seeing Ей before, but I forget... What's the difference between Ей and она?
This: https://en.wikipedia.org/wiki/Grammatical_case Which is Russian looks like that: https://ru.wiktionary.org/wiki/%D0%BE%D0%BD%D0%B0#.D0.A0.D1.83.D1.81.D1.81.D0.BA.D0.B8.D0.B9
Hmm, don't quite get it. Says she and her are different cases in English... but this is she and she...
But English has 2 (3) cases, while Russian has 7. Of course Russian will distinguish things that English doesn't. Also, I have never in my life seen any two languages whose cases are identical in function. The mistake you are making here is thinking you can translate one English structure into an equivalent Russian one. You can't. You have to make another one. This structure here actually goes along the lines “To here [Dat] • (is) needed • size [Acc] • little-smaller“. Grammatically speaking, it's the same structure as English “I am writing a letter [Acc] to her [Dat]”.