Actually, I had the same issue in the past. What works the best for me is the following VBA code. I found the linear relation by just trial and error.
The code works for single cells, but also for a selection. In the latter case, the squares are based on the total selection width or height.
Selection.RowHeight = Selection.Width / Selection.Columns.Count
Selection.ColumnWidth = (((Selection.Width / Selection.Columns.Count) / 0.75 - 5) / 7)
Selection.ColumnWidth = (((Selection.Height / Selection.Rows.Count) / 0.75 - 5) / 7)
Selection.RowHeight = Selection.Height / Selection.Rows.Count
You can put these macro's in a Module and assign buttons to them in the quick-access toolbar
Note that the squares disappear (by a changing column width) when you change the font type or size. This is due to the way Excel calculates the column width. See: https://support.microsoft.com/en-us/kb/214123