If you look at Benar Beach on the internet you can get the phone number for the site and ask the owner for the up to date situation on Electric, toilets, showers pitch charges etc.
Several mixed reports.

Benar beach is fine if you enjoy being put in a pen.

We're planning a stay there this summer. How big is the naturist section?

You can see it on Google maps.
As for being in a pen I think most naturists are grateful that a campsite caters for them.
I haven't been for a while but heard it's been extended.

Rough guess 30x20 metres could be 40x30?

It is easy comparable to a pen because of it being surrounded by a dry stone wall.  I've not used it for many years, but when I did, it was undeniably small.  As mentioned, however, it was good just to have anywhere at all. 
Following a recommendation, I began using Glynns Field nearby.  There are no facilities, but I was pleasantly surprised by the amount of naturism being practiced there, using a combination of bushes, low trees, windbreaks and camper vans / tents as screens.  I believe the foliage has been severely cut back since, so things there may have changed also.  First warm weather we get this year, I'll trot over and have a look.

I used Benar Beach many tears ago to attend a BN beach day.
Yes you are confined to a wall around you also then there was no electric hook ups and and only a thunderbox toilet. All facilities were in the textile section.
Being in the Snowdon National Park they have to comply with certain regulations.

You can see the walled in area in the right hand field.
The wall can be a blessing from the wind at night.

I've heard the new toilet block still isn't up and running.

Many I speak to at the beach use Glynn's field also but it is totally open to the road that goes to the seaside estates site.

And is opposite Benar Beach with the entrance on the road between Benar and Seaside estates

https://www.google.co.uk/maps/@52.7854621,-4.1168231,661m/data=!3m1!1e3