Spacing is how far apart the dots will be, and ‘Max Size’ is how big the largest dots will be. If you make them the same, it means the dots (or rows of lines) will touch. If that happens, fragile surface materials like paint mask or melamine will likely break off. For hard materials like anodized aluminum it’s ok to make them really close.
Max Size should not be bigger than spacing. I usually make Max Size about 10% to 20% smaller than spacing - for example, 3.3mm spacing, 3.0mm max size. If the result doesn’t leave enough surface material, you can leave the spacing, but reduce the Max size a little. ‘Dots’ mode is a little less sensitive to the spacing being too tight, because it leaves more material in general.