When I created a size attribute (for a t-shirt)… I inputed x-small, small, medium, large, x-large, xx-large. However, when choosing the product option, it reorders it to alphabetical format (large, medium, small, x-large, x-small, xx-large).
as far as I know you can reorder them via drag’n'drop. Go to Products > Attributes > Configure Terms and change the order by “Click-and-hold” into one term bar and drag’n'drop the item to any position you like. When you click on an item you’ll notice that the cursor turns into a “cross” which indicates you can move the item now.