joking of course
when central banks sell billions the price moves. At a minimum it would i would guesstimate to be in the hundred's of millions. Depends on time of day liquidity etc. A $5million ticket is nothing to the market. Volume $1.5T+ per day.
with 25-30 million $ go on USDPLN or EURPLN you'll impact the market to the like with a little help of some dealers who just long for such happenings. But the timing needs to be carefully selected.