My medusa swelled horrendously. It may be that I just have a thick top lip, but it swelled to the point where I needed to wear something with a 5/8" wearable area for the first few weeks. Now, I need 7/16" wearable area, 7 months later. It's just a thick part of your lips. I know swelling will be different for everyone, but I definitely think 5/16" is way too short for initial swelling, regardless of who you are.

most fresh lip piercings use about 1/2 inch wearable ((as far as i've been told by my piercer)) that part of the lip is pretty thick, but its thinner as you go higher up on your lip.  you should have had your lip measured before purchasing, but look at it this way: now you have a retainer for when its healed.  go get it measured, and if you cant find the right lentgth in glass, you can buy a bioplast barbell, and a disc for one end, with w/e tip you need for the front (clear w/e) 

plus, having a piercing in the middle of your face is really hard to hide. even a little clear retainer is pretty obvious.

just throwing it out there.

Mine swelled up a lot. As for the retainer/glass labret you wont be hiding it, anyone up close will notice it. Especially clear glass from my experience with it will reflect light making it more pronounced.