I live on the beach in Northern California and had the same question as you. The first bike I bought had 3.8" tires and it did not work at all in the sand. So I went to 4.8" tires. It was still tough but worked. Then one day I let almost all the air out, I was down at like 3 psi, and boom! Everything changed, I could float up and down the dunes with ease. I have a 1x12 Sram gearing, which has a very nice low granny gear. Super low tire pressure and a granny gear and you're good to go.

All the above is based on dry loose sand conditions, but if you're lucky enough to catch a ride when the sand is wet or was wet recently, you could pretty much ride a bmx bike out there and be fine. The sand changes dramatically when moist and it becomes a breeze. For those conditions I pump the tires up a fair bit more. Happy trails!

PS- Search "Sequoyah Faulk Fat Bike Dune Bombing" if you want to see some footage of me riding the dunes here...