If you want 50 pips to equal $2, then each pip must be valued at 4 cents. ( $2/50 = $0.04).
If you are trading a currency in which yours is the counter, you need a position size of 400 units (0.4 microlot).
Trading 1 microlot (1,000) EUR/USD in a USD account sets the pip value at 10 cents. So to lower that value to 4 cents you need to trade 0.4 micros (400).
In what currency is your account denominated and what pairs are you looking to trade?