How to combine matching records while summing a field?

I am creating a report that displays records along with a dynamic path image. I would like to combine records that have the same size, and sum the amount of units. For example: If both records are 36x36 and have 2 units each, I would like to display one 36x36 record with a unit count of 4.

I have tried the group expert but that doesn't seem to combine them, only separate them by size with a unique header.

The report:

You were in the right path, but need to move the images to the group section. Try again like this:

1. Create a group by size.
2. Move the images from details section to the group header (or footer) section.
3. Create a summary field to sum up the units in the group footer section.
4. Actually, I understand you will need to move all from details section to group footer (or header) section.