Thanks for the suggestion. We agree that it would be useful to have the size vary based on the data and we will think about how to add this. This could actually end up being a new command where you have all sorts of options that would vary for a label (e.g., size, color, rotation, location).
For now, we added an example file (File/On-line examples) with a technique we have used to get variable label sizes.
In the example, one command is drawing all the points with no labels. Three additional Points commands are drawing the labels, using the ‘No Point’ option. Each of these commands has a mask to only draw labels depending on the value of the data. Each one uses a Font variables with varying sizes.
This does not allow the font to vary with a column, but at least you can set up some discrete Font sizes to scale with your data.
Hope this helps for now. We will work on an option where you can use a column to vary the font size, color, … in a future release.