Resize windows while maintaining proportions in OS X
Change the size of windows while maintaining current aspect ratio in OS X
Here's how you can resize windows proportionally in OS X.
1. Hold the Shift key while resizing - This lets you maintain the current aspect ratio.
2. Hold the Option key while resizing - This lets you resize the window from the center.
3. Hold the Option and Shift while resizing - This lets you do both of the things listed above.
For more, see the original article at the link below.