Lock/unlock aspect ratio while resizing shapes in Lucidchart

Hold Shift while resizing. If you resize diagonally it will keep both horizontal and vertical ratio. If you resize horizontally it will keep the same height; if you resize vertically it will keep the same width.

Edit: We also now have an option to lock the ratio just go to Shape Options on the toolbar at the top and click the lock icon between the width and height.  Here's a screenshot of what I'm talking about.