In readme file https://github.com/cathalmccabe/PYNQ_tutorials/blob/master/dma/dma_tutorial_part1.md
section DMA read and write channels
It reads: Set the stream data with to match your IP stream width. In this example I will leave it set to 32.
The image shows 64

Image location: https://github.com/cathalmccabe/PYNQ_tutorials/raw/master/dma/images/dma_config.png