I) Maximum Trade Size is being checked out whenever you put an entry order (Market Order or Pending Order), for instance:
1.1. You have an open position 1.0 lots EURUSD and wish to buy 1.0 lots GOLD. Such request will be confirmed, as Maximum Trade Size is 2.0 lots.
1.2. You have a pending order Buy Stop 1.0 USDJPY and wish to buy 1.0 GBPCHF. Such request will be confirmed, as Maximum Trade Size is 2.0 lots.
1.3. You have an open position 2.0 lots GBPUSD and wish to buy 1.0 lots EURUSD. Such request will be NOT confirmed, as Maximum Trade Size has been already reached, and 'Invalid Volume' message will be returned on your request.
1.4. You have an open position 1.5 lots GBPJPY and wish to place a Sell Limit order 1.0 lots USDCAD. Such request will be NOT confirmed, as Maximum Trade Size is 2.0 lots, and the resulting summary would reach this value, and 'Invalid Volume' message will be returned on your request.

II) Maximum Trade Size is not applied to hedged positions or pending orders intended for hedging the existing position, for instance:
2.1. You have an open position Buy 2.0 lots EURUSD and wish to sell 2.0 lots EURUSD. Such request will be confirmed, as Maximum Trade Size is 2.0 Lots, and Hedged Margin is 50%.
2.2. You have an open position Sell 2.0 lots GBPUSD and wish to place a Buy Limit order 2.0 lots GBPUSD. Such request will be confirmed, as Maximum Trade Size is 2.0 Lots, and Hedged Margin is 50%.
2.3. You have an open position Buy 1.5 lots GOLD, and wish to place a pending order Sell Stop 3.0 Lots GOLD. Such request will be NOT confirmed, as Maximum Trade Size is 2.0 lots, and the resulting summary would reach this value, and 'Invalid Volume' message will be returned on your request. 

III) Maximum Trade Size is calculated for certain markets using specific multipliers, as follows:

SILVER, Indices - x0.1
Stock CFDs [USA] - x0.001

For instance:
3.1. You have an open position 1.0 lots USDJPY and wish to buy 5.0 lots [DJI30]. Such request will be confirmed, as 5.0 lots on [DJI30] will be calculated as 5.0 x 0.1 = 0.5, and Maximum Trade Size is 2.0 lots, so the resulting summary (1.0+0.5) would not reach this value.
3.2. You have an open position 10.0 lots sell [SP500] and wish to sell 3000.0 lots #IBM. Such request will be NOT confirmed, as Maximum Trade Size is 2.0, and the resulting summary will be calculated as 10.0 x 0.1 + 3000.0 x 0.001 = 4.0 and would reach the Maximum Trade Size value.