I recently purchased a NIB Alpha trainer. This is the red, white, and blue covered version before Horizon discontinued it. I plan to setup with a brushless motor. The instructions say to use the E-Flite 25 outrunner. However, Horizon's current website shows the E-Flite Power 46 outrunner being the choice for the Alpha trainer. Which outrunner should I use?

I'm not familiar with the model but a web search reveals there was more than one version/size of it out there. I found a .40 AND a .60 size so it is possible there was a .25 size. So, I would suggest you measure it. The wing span should be 63 inches and the fuselage 52.5. If so then you have a .40 size model. It's ready to fly weight should be in the 5.25 range. If it is smaller than this then you might have a version intended for the .25. Bigger and it's the 60 size. Good luck!

I definitely have the .40 size. The pictures on the box show the E-Flite .25 outrunner and the instructions specifically reference the E-Flite .25. My general question is - is a .25 outrunner electric motor adequate for a conventional .40 size balsa trainer? This plane can also be assembled with the Evolution .46 glow engine.

Most .40-size trainers, the Alpha included, will actually fly "adequately" with a .25 brushless outrunner or .25 glow engine. Moving up to a .32 electric/glow or .40 electric/glow gives the pilot a wider performance range and more power for windy-day flying. Most pilots would generally advise a .40/.46 brushless setup for an Alpha .40 trainer so you can throttle up if you need to penetrate a head wind or if you want to learn basic aerobatics after getting comfortable with basic flight.

An Alpha .40 trainer being flown with pontoon floats or skis might actually fly best with a .52/.55 power setup (electric or glow). The suggestion of .40-size power is simply a likely sweet spot over a wider range of possible power options.

Good luck and happy shopping!

I contacted Horizon Hobby about this and was told the E-Flite Power 25 is the recommended outrunner for this plane. Interestingly, Horizon's website says the E-Flite Power 46 is a good match for the Alpha Trainer. Seems like some contradiction there.

I know there is a wide range of powerplants suitable for the Alpha

What many new hobbyists don't understand about electric motors is that the power they make is dependent on the load you put on them. It's backwards from the way internal combustion engines work. IC engine make power, then you chose a prop that is appropriate for the torque curve and power of the engine. But with electric, the prop determines the power. You can prop down and reduce your amp draw, allowing the use of smaller batteries. The only downside is you've installed a heavier motor than you needed to. Then if you need power, a prop change can increase it 20%. You can keep going until you exceed the amp rating of your motor or ESC.
So all that is to say that it really doesn't matter. The .25 equivalent motor will be at its capacity turning the right prop for that plane, while the .40 equivalent will probably have some headroom left.