Theories in order of likelihood:
1. Drugs that increase size for a short period of time. This drug would most likely have a side-effect, meaning it can only be used now and again.
2. Spirit Sex. Many believe in Astral Sex, or sex during Astral Projection. Here's one such person:
https://www.erinpavlina.com/blog/2012/03/astral-sex-the-ecstasy-of-out-of-body-pleasure/
Elbaph is already weird enough where this wouldn't phase me.
3. Magical realm in Elbaph that lets people shrink down to the same size. Think Alice in Wonderland.
4. Homunculi clone. We know cloning exists in One Piece; what if Elbaph has the ability to make holograms, like in Egghead, through the Sunstone? IE, make a hologram of yourself into a giant, and then have sex.
As someone who is really interested in sex through Astral Projection, I'm the most interested in seeing that.
Whoops, forgot one:
5. Giant Prosthetic Penis that you insert your member into in order to control/feel.