If you’re drawing 5x10 bricks at 10 pixels/mm, then they’re going to come out half a millimeter tall and one millimeter wide.
If that’s not the size you want, then draw the bricks at the size you want, filling 10 pixels for every millimeter. If you want a 2 mm × 4 mm brick, it must be 20 pixels tall and 40 pixels wide.
It really is that simple!
Remember, the reason for 10 pix/mm was to match the engraving line spacing of 0.1 mm.
If you use 72 dpi, then each pixel is 0.353 mm on a side and the engraving line spacing must be 0.353 pix/mm. That seems rather coarse, but if it works for you, go for it.