I've used polygon, with the grid used as a guide. With Ortho off, diagonals can be made.

Make sure no layer is frozen. Otherwise copy won't do anything.

Open your map; click on FS: which is the fill styles dialog.

Click on the bitmap fill you want to scale.

Towards the bottom you will see a line with:

Scale [] Width [a number] height [ a number]

Click on the scale box so it has a check mark in it.

Put the same number in the height and width boxes. Click okay. All done.

If you also posted on the fb page, I gave a quick way to do it.

I'll post it here.

Freeze all layers except background. Change the color to color 15.

Go back to the layer dialog, Freeze all except the grid layer. Change the color of thd grid.

If the background doesn't change, you will also need to changefs to solid.

I freeze all but walls. If the wall is straight and level or vertical, i click on Ortho bottom right.

Then I use the Node Edit tool on the left to select and drag the wall so it closes the opening.

It may take some care to accomplish.