On most of the ECN platforms (IB, MBT, Currenex, HotSpotFXi, FXAll, ChoiceFx) you can easily trade in and out 1MM to 5MM w/o affecting the market. In fact, except for MBT and IB, most of these platforms kinda discourage trading sizes less than 1MM. But with these platforms, you can see the bid/ask depth to determine what could be filled with ease (i.e. it is easier to fill EURUSD in size than USDNOK). If you are managing OPM, then these platforms should be more than sufficient for your means.
If you need to push larger sizes (20MM+) regularly, you will probably need a tier 2 access level platform (i.e. credit line at bank on their platform).