Merging faces within a flat object

Maybe a simple question, but I haven’t been able to find a simple answer to it online.

I made a mostly flat model by stacking (a lot of) cubes and then merging them. Looks like the faces on those cubes did not merge to form one face like I intended. It is flat, but I need/expected the model to be much smaller in terms of file size than it currently is.

How can I merge the faces? I’ve read that holes in the faces may be an issue and I can always re-merge those if necessary and do them a different way.

If they were cubes there may be faces inside the mesh, delete them
(delete doubles and use Select / Interior Faces)
X gives you various delete options (Limited dissolve, dissolve vertex etc)

You’re the only person with the blend file so you’re the one who’ll have to test which options are best

Thanks, I managed to delete all of the interior faces. I still don’t know how to merge the external faces.

None of the various Dissolve options seem to do anything other than remove all of my selected faces regardless of whether i select faces/edges/vertices.

I just want a single flat face where all of these tiny square faces are. Dissolves dont seem to work. I tried deleting the faces and selecting all of the edges around the hole and filling them and that only filled in a corner.
Hope helps.