Lock aspect ratio of objects?

Hi there

I am working on  a question slide that is drag and drop. I am wondering if there is a way to set the aspect ratio of the shapes I am using for the drag and drop so they are all the same size? Each shape is slightly taller or shorter. I don't want to re-size to fit text, but rather make all the rectangles the same size. 

See attached screen shot (due to the nature of the lesson content I had to block out the text)