Dear BigH,
To set the width and height of cells so that both be equal in size, you should set the height about 6 times the width.
It need some trial and error to find the no. of pixels required to make 5mm length.
Farzad
I have used Excel many times to prepare block flow diagrams. Holding down the Alt key while drawing lines, locks the ends of the lines to the grid. Make blocks cover an even number of squares then joining lines can enter the middle of the the blocks. Use copy and paste to replicate blocks and position them by holding down Alt. Use a shading effect to distinquish blocks from lines.