DonnaPea, I use a tool called a Bias Edge Square. You can trim from 1/2 sq in" triangles up to 8" triangles. I never needed anything bigger. But, I believe the tool does come in a 12" square also.
You might want to look around and see if you can find one or something similar. Even JoAnn Fabrics might have something similar that you could buy with a coupon. I find this tool to be my most highly prized possession in the sewing room. My half square triangle blocks are almost always spot on when using the tool.
My 8" Bias Square by the Patchwork Place in action.
Example of tiny trimming. This is a 4.5 x 6.5" postcard. The 1/2 traingles measure 1.5" sq. All trimmed with my bias sq. ruler.
