How far is the walk between the Thai and Malaysian immigration checkpoints at the Sadao/Dannok border?

There's a small parking lot on the right, 20 meters from the checkpoint. Then walk 50 meters to the Malaysian checkpoint, they'll stamp your passport, and head back.

A small additional bit of info: Sadao crossing is a huge border with lots of people. Wang Prachan border is very, very small with almost nobody. The last two times I went there, there were just a few Malaysians on a road trip crossing with their motorbikes — and me, the only farang (foreigner) ^^

Yes, they're very chill there and unlikely to give you any trouble or ask many questions. But they did ask me for a 2000 THB fee because I told them I was just there to reset my DTV and wouldn't be spending the night in Malaysia. Their response was that there's a "new rule": if you come back the same day, you have to pay a fee. I suspect it's tea money, of course, but I didn't want to make a fuss about it. For info, I've been resetting my visa here for many years and never had to pay before — but back then I had a Non-O visa, not a DTV.