Long gone are the days where the market waits to find out he size of a rate hike/cut. The current regime clearly communicates these things ahead of time and takes what the market gives. Right now, 25bps has been communicated and also priced-in by the market. That won't be a surprise.
Less obvious, but still likely surprising to some will be the way Powell delivers the message in the press conference. Hopefully markets haven't confused a "smaller rate hike" with a softer stance from the Fed. Powell's message will likely reiterate that fact. The big question is whether the average trader appreciates and understands the Fed's stance. They probably do, based on the reluctance to challenge the prevailing range in the past 3 weeks, but we'll find out for sure after 2:30pm ET.