Can't say that I've ever had a mental picture for what Helen is supposed to look like, Nasuverse or otherwise, because she runs into the exact same problem as Aphrodite.
Aphrodite's thing is that she is attraction (as well as a bunch of other things).
However, you start straying from that as soon as you start providing physical descriptions because different people have different notions of beauty. Never mind the differences that can exist between different times and places. If you stick to text, you can get away with descriptions that suggest attractiveness without actually having to mention any physical characteristics. In contrast, if you move on to visuals, well, good luck with that.
In other words, I don't have enough imagination to even start envisioning what the most beautiful woman in the world is supposed to look like.
So, going by how Stheno and Euryale are supposed to be "idol goddesses", that means both Helen and Aphrodite were lolis.
If you go by what comes up in the story of Artemis and Aura, Artemis is supposed to have an 'Aphrodite-like' body...
... and part of Aura's mocking of Artemis' body involves describing Artemis as having large, heavy breasts (enough so that Aura openly questions if Artemis
really has never had mother's milk developing in them) and slender arms (in contrast, Aura claims to have an 'Athena-like body', with a 'boyish chest' and visible muscle definition in her arms).
Though in the Nasuverse, according to Orion's artist I-IV in the F/GO Materials entry for Orion, the plan is for Artemis'
true form to be 'neither busty nor loli', but Artemis
has both a busty avatar (seen attached to Orion's Saint Graph) and a loli avatar.
Helen of Troy was Spartan.
She obviously looked like a gender flipped Leonidas.
I seem to recall a mention that Helen was supposed to be on-par with her brothers in combat skills in her younger days.
No, it was Theseus who kidnapped Helen when she was ~10 so he could keep her around until she was old enough to marry. Only after the Dioscuri rescued her and she grew up to marry Menelaus did Paris pull his bullshit.
Ah yes, Theseus' and Pirithous' idiotic plan to score 'daughters of Zeus' as their wives. And if you thought Theseus' side of things was stupid, Pirithous was going to kidnap
Persephone from the underworld to force her to marry him.
Naturally, Hades was
not amused. He eventually let Heracles take Theseus away from his imprisonment, but Pirithous? Hades was not letting that attempted homewrecker go
anywhere.
A couple incidental details I found regarding this incident: from what I see, Helen was 12 when Theseus kidnapped her. Also, the Dioscuri, while rescuing Helen, took Pirithous' sister Physadeia and Theseus' mother Aethra captive and made them serve as Helen's handmaidens.
