Baldeagle puts forth some excellent points. Overall, installing a 50cc in your pride and joy is overkill x 10. (this is an extra point that I am adding in addition to Baldeagles
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For regular non 3D, "you just want to have some fun.. A 26cc will respond nicely in the bird. For some real "hold onto your sticks performance" that includes 3D and stuff you would never do otherwise, consider a [link=http://www.zrcgf.com/store/Details.cfm?ProdID=28&category=0]45cc engine[/link].
The [link=http://www.zrcgf.com/store/Details.cfm?ProdID=28&category=0]45cc engine[/link] size will provide all the craziness you will need or desire in your Extra. It will also fit better into the cowl area, and it will NOT hang out underneath as much as the larger size engines would. Looks matter too!!
Another concern with using a larger size cc engine that many guys don't figure upon until the guts are installed, is the prop clearance. You gotta has some of that, especially for those less than TOC type landings we all strive to do.[X(]
The whole purpose of using larger size engines in these types of planes (aerobatic and highly maneuvourable ) is NOT for speed per se, it's for the power needed to get INTO and OUT OF maneuvers that require the horsepower to PULL the plane when the airflow over the wings is not there.
Best ta all!
Henry