If you're producing something that only has one size or is adjustable to fit all sizes you won't need an entire size range built out. Here's how you manage this scenario:
Make sure your product / block has already been added to the library
Enter your product summary or block view
Then click on Size Specification from the left-hand menu if you're in the product summary view
Now, click the 3 dot icon in the top right hand corner
Open up the option for Edit Size Spec
Scroll down and toggle the option for Alphanumeric
Add your text "One Size" / "OSFA" / etc. to the text box, hit enter, and save.
When you're done you can then add points of measure from the search bar to begin building out your table.