What size engine was standard for the 71 Chevelle SS?

I think they came with a 350, 400 or 454.

which ones were big blocks?

The 400 were actually 402 big blocks, but chevy called them 400's. These were available in SS or Malibu cars. The 454 is a big block and only available with the SS. Of course, the 350 is a small block.