There are two methods of resizing cosmetics. Step 2 is the easier method.
Find an entry in the JSON text that already has a "Size:" portion. Edit the value from between -2 (smallest) to 2 (largest). If the entry has "Spacing:" or "Angle:", set those to zero.
Keep in mind that you should double check what item you're replacing, as an entry in the JSON text that already has a Size: portion is likely to be another pair of lips which will cause the disappearing soft glitch.
Manual glitch: if you know how to do the manual glitch in shop using a nose, then you can do that to your soft that you just placed. Click a cosmetic to purchase in shop, and then go to your inventory and click the cosmetic you wish to resize. Click on your avatar and open the resize tool for the shop cosmetic, and it will allow you to resize the softed cosmetic. When you save your outfit, the softed cosmetic will stay resized permanently.
The manual glitch is shown in this tutorial at the 7:15 mark.