IB has numerous market makers and a summary of the bids/asks is available through their "Market Depth". The amount of currency on each side varies greatly by the currency pair and time of day. For example, right now (20 minutes before the daily forex close), I'm seeing for EUR.USD about 30 million Euros on each side. The spread is 0.5 pip with 3M x 3M, at 1.5 pips it's 15M x 19M and at 3 pips it's 12M x 2M. It sometimes changes multiple times each second.