Can easily trade $20 million in equities, though really is a function of leverage and desired return on capital. If offered 10:1 leverage on $2 million, that's $20 large, minus the margin charge.
As liquidity is a necessary prerequisite, otherwise slippage becomes a real haircut, IMHO best to trade black box spreads of at least 50 to 100 NYSE listings. If flat rate commission, returns can be stellar. How much? 200% to 500% per month on original capital, i.e., the $2 million and so on, as per each month's starting equity plus profit additions. Not a bad progressive pay day, year in, year out, other than short term capital gains tax unless trading for non-USA client(s) offshore.
Now, want to trade someone else's $1 billion for a 25% of profit cut per annum? Not out of the question . . . in fact, it's reality.