Good work Simon and all of the rest of you, too.
Neil, the size of the square doesn't matter.
Components and the Hide function of the eraser toolare the keys to this one. Break the entire piece down into small components and leverage the fact that copies of components don't appreciably add to file size. One thing I didn't see mentioned is that you should also delete the faces that aren't needed--the ones between components. Faces are entities that add to file size.
The other thing that Simon mentioned is the Make Unique operation. This allows you to get away with only drawing the first component. You modify the edge components as needed.
When I drew this for an actual project a while ago, the size of the opening and the width of the wood between was called out. It was a commercially available thing. The opening it had to fit in was also specified. To make the gridwork look right in the space, I started with a component at the center of the opening, the dark green one in the picture. Then I copied out to the left and to the right to the width of the opening and just beyond. (Ctrl+Move to make a linear array). I select the entire row and copied it up and then down. Then I started working on the edges. Selecting those components that would be related, making them unique and editing them to trim them. In the end there have to be 3 different components for the grille.
In the picture I've invert one each of the three and removed their back faces so you can see the faces that were removed on the seams.
I've written before about making symmetrical things using this technique to reduce file size. If you remember the pipe clamp model, I did that splitting the cast iron parts in half left and right. The Acme threads on the screw were drawn using a three-sided components with its edges hidden. the entire clamp saved as 138Kb. I could reduce the file size further by eliminating the faces that would be against the pipe.