The global currency post market turnover is ~1.4 trillion according to the BIS, note this includes many currency pairs that dont involve USD; a trillon a day is an upper bound on the daily USD market.
China effectively traded around 0.2 trillion USD for yuan and kept the USD last quarter, the impact is not that big.
More generally looking at the volume transacted does not tell you much about the depth of the market; you need to look at the order book for that. Looking at the bitcoin market there are several lots of over 25k sitting within 20 cents on each side of the bid-ask, this means it is extremely likely one could carry out a directional 100k trade with relatively small market impact if spread out over a few days.