Regular size portable washers (like the one I have) take about 1/3 of a regular load of clothes or one bedsheet plus 1 or 2 pillow cases, depending on the size of the sheet. And Outlander's right, you can't wash blankets in them, and only a couple of jumbo sized bath towels at a time.
They all have a spin cycle, as far as I know. They don't spin as fast as regular washers, so you still have to wring some water out. Mine wobbles while it spins...they're just not built to take the centrifugal force. But it's not a big deal.