This function helps to calculate the lot size needed for any position where the entry price, initial stop loss price and amount being risked is all known. The function returns the amount according to the number of decimals allowed by the broker - generally speaking, if mini lots are allowed to be traded this means you can have 2 decimal places.

getLotSize.mq4

/* 
 * Calculates the position size of the trade based upon entry price, stop loss & amount risked.
 * @param entryPrice double  Entry price of the order
 * @param iniStop double   Initial stop loss of the order
 * @param riskAmt double   Risk per trade (in dollars of account's currency)
 *
 * @return  amount in mini-lots
 */
double getLots( double entryPrice, double iniStop, double riskAmt ) {
    string sym = Symbol();
    double stopDist = MathAbs( entryPrice - iniStop ) + ( MarketInfo( sym, MODE_SPREAD ) * MarketInfo( sym, MODE_POINT ) );
    double lots = ( riskAmt * MarketInfo( sym, MODE_TICKSIZE ) ) / ( stopDist * MarketInfo( sym, MODE_TICKVALUE ) );
    return( N( lots, 2 ) ); // this will round up/down!
}