When a product has more than one option set, you decide which set comes first by giving each one a number. That number is the Sort order.
The smaller the number, the higher up the product page the set appears.
1 is shown above a set with 2.2 is shown above a set with 10.So if you want your customers to choose the size before they see the add-ons, give the size set 1 and the add-ons set 2.
The app sorts in two steps, always in this order:
Product pageSet A · sort order 1SizeColourSet B · sort order 2Gift wrapGift message1. The sort ordernumber decideswhich set comesfirst.2. Your order insidethe set decides theorder of its ownoptions only.
A set appears as one solid block. Options don't leave their own set to sit among the options of another one.
This is the part that trips people up, so it's worth saying plainly: if set A has a lower number than set B, then every option from set A appears above every option from set B. It makes no difference how the options are ordered inside either set — that order only ever applies within its own set.
One exception: if you have rearranged the options by hand on an individual product, that product can show options from different sets mixed together, until the next time one of its sets is saved. See Change the order of options on a single product below.

They're placed one after the other, and the set you created most recently goes on top. That means adding a new set can push an older one down without you changing any numbers — so if the order matters, give the sets different numbers rather than relying on this.