The ‘Tile Size’ setting is only available when the dashboard ‘Sizing Approach’ is set to ‘Reflowing’.
Upon the introduction of Scalable Dashboards, the scalable option was made the default. If your goal is to get smaller tiles and you have the Scalable dashboard sizing option selected, you can increase the number of columns.
So if you currently have a 6 column wide dashboard, you could change it to 12 columns wide if you wanted to double the number of columns and thus make the base tile size 1/2 of the original size.