The correct procedure for this would be to place the Tiles with different unit sizes into different Tile Palettes. If this is required, you can manually change the Grid's Cell Size by editing the Tile Palette Prefab and changing the Cell Sizing Palette Setting to Manual.
If Tiles with different unit sizes that differ by 100x or more in scale, having a deeper zoom level can help in identifying the Tiles, but will make usability a big issue, as you would need to scroll more to select between the Tiles with different sizes.
To help identify the Tiles better, we will work on adding a feature which will show a Tooltip or an Info box to describe the Tiles you are hovering on the Tile Palette.