It would import only the cells which have data, then you can have 2d arrays with varying indexes such as:
[1,1]
[2,1][2,2]
[3,1][3,2][3,3]
[4,1][4,2][4,3][4,4]
[5,1][5,2][5,3][5,4][5,5]
Why import empty/unused cells? That would be totally unnecessary. But the nice thing is that being able to manage data outside of Stencyl in a spreadsheet would be amazing.
I mean suppose you have certain formulas and calculations you want to perform on specific data that would be added in via a list (whether single or multidimensional), you can do so within a spreadsheet editor.
Then if you need to tweak certain values, you can make a few simple changes in the spreadsheet editor, let the formulas update, and re-import the .xls into Stencyl with the updated values. So this makes not only the importation, creating, and management of lists easy to work with in stencyl, but it also makes it easier to create and manage them outside Stencyl with more powerful tools.
It's like Stencyl allows you to use a 3rd party image editing program by clicking a button, you can code with a 3rd party program, etc. Why not create lists this way also, at least for the more advanced needs, and offer a basic editor for the most general cases?
Edit: And sunflower, unless Vectors were being used in Stencyl rather than Arrays, you wouldn't have to worry about fixed sizes. Arrays will grow and contract based off whats added and removed, which is exactly how lists work. But vectors allow you to specify an absolute size and data type to work with, so if you intend to have the list as a point of reference and it is not something that dynamically changes, then Vectors could still be supported under this method. It would just mean that whatever data is imported via .xls is exactly how big the Vectors will be, considering the classes are generated at compile-time.