Yes, you can reduce the size, but be careful with the seam allowance. In general, you remove the seam allowance, resize the piece, then add the seam allowance back on.
For instance: If the pattern tells you to cut a 5" square, and you want to make it twice as big, you would remove the seam allowance by taking 1/4" off all the way around, so that would reduce it to a 4.5" square. Double it, giving you a 9 inch square then add the seam allowance back on resulting in a 9.5" square.
After having said all that, triangles are really, really, really tricky. What you are describing sounds kind of like a squared triangle block. I would just skip the calculations and get a tri recs tool. Here's a video:
http://www.youtube.com/watch?v=8CT7g1NXrL4