How does £1,000 in dividends every month from a Stocks and Shares ISA sound? The good news is that the maths shows us this target is entirely achievable, assuming someone is consistent and patient. Here, I'll show how large an ISA would need to be to throw off a grand a month in tax-free passive income. The average dividend yield of the FTSE 100 is currently a smidgeon over 3%. A booming bank stockTurning to Footsie dividend stocks to consider, NatWest (LSE:NWG) strikes me a solid contender.