IC Markets out of Australia seem to be the largest globally. Also, it might be leverage generosity with an A+ rating. Stick with Oanda as they still are large for the USA market.
Global by region
http://www.wvwv.org/forex-brokers/largest-forex-brokers.html
Larger brokers for USA
It might be wise to stick with Oanda for both the current Python API and even MotiveWave platform !!
NOTE I now post my TRADING ALERTS into my personal FACEBOOK ACCOUNT and TWITTER. Don't worry as I don't post stupid cat videos or what I eat!