If they haven't released any official statements regarding recommended specs I think your best bet is to use Divinity: Original Sin 2 as a comparison.
I would add a decent amount on top - especially now, D:OS2 got some decent amount of optimisation, and I don't expect BG3 EA, or even 1.0 to get same amount of love, not to mention that higher visual fidelity will most likely come with higher requirements as well.