One way to do this is to create a lookup table to pull your size drop-down values from. In the lookup table, make a 'label' and 'sortvalue' column and assign your values accordingly (xs = 0, s = 1, m = 2, etc). When you're setting up the datapage field, you'll set the field type to a drop down and use your lookup table as the source. You will be given options about what field do you want to determine the sort order - pick your sortvalue column from the lookup table then set your ascending/descending setting to your liking.
That should do it, let us know if that works.