Almost the same size is not good enough

Making similar boxes the exact same size, and exactly aligned matters a  lot in slide design. The brain gets distracted when object alignments is just a bit off.

Usually the slide starts out OK, ctrl-C/ctrl-V a bunch of objects and they are all exactly identical. Over time, things start to degrade. Accidentally resizing things a bit, moving a box a bit, etc.

You need to train your eye to spot the imperfections. The quickest fix is usually to select a group of objects, select "format" and then give them all the same size in centimeters (hight, width, both). In the Arrange / Align menu you will tools to spread objects out evenly.

Little effort, big result.