Please include the output of the power plant next to, or in lieu of the power adjustment on the power plant design screen. This way, you will know immediately how much power a specific size of plant will generate, without having to click on each one.
If you have your cursor on the selection box you can just scroll through the options with the middle wheel, which usually works just fine. Much easier than laborious clicking.
There's actually a technical reason why the selection box can't be done this way: the inputs to the selection box are static (I believe hard-coded) and don't change with the type of power plant/reactor technology. This is the same as every other selection list box in the component design window works. Recoding these to be dynamic would be a ton of work, and the numbers are displayed in the component text box anyways so it's not really needed.