Michele, to create a triangle, you can use the Selection tool set to the Point-to-point, like you probably did, or you can also create your own preset shape which can be sized as you wish, without distortion. One trick, if you want to create an even triangular selection though would be this:
- create a SQUARE selection
- change to the point to point selection mode
- hold the CTRL key (to remove) and select from one corner to the opposite and then all around the part you want gone
Now, your selection should be exactly what you wanted. And then, using the Edit selection command, you can flip and mirror your selection to either apply an effect or fill with a color to be replaced later (as a placeholder).
This might become a tip in the Tips and Tricks section!