The Size input for Marks is a slider.
I want to input precise numbers so I can synchronize mark size across worksheets on a dashboard (for example, bar charts).
Fine control with the slider is imprecise, and small differences in mark sizes presented as comparable information (example, revenue by state next to revenue for top 10 customers) can distract the viewer.
I see this as analogous to specifying the % Transparency in Marks Color (which also has a slider) or the pixel position and size for floating objects on a dashboard.
I couldn't find this as an existing idea, but at least one other person has had it: