I phone my orders to my broker. When i tell him to sell 100 GBP at the market, that actually means sell $100 quadrillion GBP...
Not possible. Even if you had 30 10 billion$ accounts (with different prime brokers/banks etc) and simultaneously (say within 3 minutes) bought or sold $200 bill (@20:1 leverage) in cable - you would seriously not be able to offload such a trade unless you moved the market 100 pips (remember cable has about 250b.$ a day turnover (half of it in spot). Soros did something like this is '92 (smaller scale) and if you thought that cable was going to surge/tank (500-2000 pips) then it would be worth it.. In this situation I would set up (or buy) my own bank and credit lines and get EBS/reuters accounts for myself.
I cant reveal all my methods on ET (incase other traders steal some of my liquidity), but there are ways of doing this kind of size without much slippage.
I did it last week when I was learning how to trade. I can teach anyone how to do it for a measly 10.000$. GS