The Secret of the Shift Button:
In Google Drawings, you can resize a shape, text box, or image fairly easily. Just use the points on the outside of the bounding box to resize the object in whatever way you want. However, if you'd like to keep the item in scale, you can press the shift button while resizing with the corner points. This also works across into Docs and Slides when you are working with text boxes.
Also, if you are rather OCD like myself, you hate drawing a line and noticing that it isn't quite parallel or perpendicular to the grid. The shift button also allows you to draw lines that will always give you perfectly parallel line.
Simply start drawing your line, and hold the shift button as you finish your line. You will notice that the line will only go straight across, not up or down just a little. This will also work to resize a line, or if you drew a non-parallel line, you can hold shift and adjust it as well.
Now let's say that you are trying to find the exact same angle every time. As you are holding the shift button and moving a line, it will automatically move in 15 degree increments. You can see my example below (I am holding down the shift button the whole time as I move the line):