Heh. Enjoyable responses, guys. To answer the question, it was like the size of a fruit fly, I guess?
In the end, our heroine–oh, screw the embarrassment–I chose save the fly, got some water instead to slake my thirst, waited a couple of hours angsting, and then said fuck it and drank the juice. And it was good.
Now, to those who say it’s likely impossible to save the bug anyway…
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Choie’s Method for Saving Bugs in Liquid:
For those of you who might be curious, which I suspect is very few, here’s the trick I used for the fly. Being a bug-saver from way back (seriously I’m like some kinda Hindu benefactress of the insect/bug realm*), I’ve developed a few methods because I just cannot bear the thought of seeing something die, much less having inadvertently caused it by… I dunno, existing.
If a bug/insect is swimming/floating alive in liquid, you take either a very thin piece of paper, or better yet a thin paper napkin, and place it gently very near the fly (or whatever it is). The liquid’s surface tension is likely what’s keeping the bug afloat. So what you want to do is very carefully break through the surface tension as shallowly as possible, and as close to the bug as possible without actually touching it. This way, the bug is sorta drawn into the napkin along with the tiny amount of liquid. The bug crawls up onto the napkin now that it finally has some purchase.
Take the napkin out of the glass/bowl/whatever and lay it down somewhere so that the bug is on top (obviously, or you’ve just wasted your rescue efforts). If the liquid is thin/watery, it’s likely that the bug will crawl or fly away. If it isn’t, it may need to dry off a bit first.
This method has helped me save little moths who’ve flown into liquid, which is pretty impressive since they’re so incredibly delicate. With them, you have to set the paper aside and let it dry, as their wings need time to recover.
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Before anyone says anything, I know how crazy neurotic this sounds. I am very, very aware that this is more effort than a normal person would go through, and I’m a freak worthy of mocking. I cop to everything. I’ve actually explained my agita when it comes to bug-killing before (it doesn’t help that I went through a period of depression–well, I still have that, but it’s being controlled–where I was a severe hoarder/clutterer and bugs were rife in my apartment; severe trauma occurred when I knew I had to clean and that meant facing the fact that it was either a clean apartment/dead bugs or a disaster area/bug haven).
The fear of dealing with that Sophie’s Choice dilemma is a big part of what keeps me from reverting to my messy ways.
This extreme over-empathy for creatures gets wearying and makes life difficult at such times. I don’t want to look at/see bugs because, y’know, they’re icky and unhealthy. Plus they scare me. But I live in NYC and, well, bugs are gonna be visiting at some point or another. (Although I’m happy to say that despite living in a building right above a popular restaurant, I’ve never seen a cockroach in twenty years of living here. The main crawly-things we’ll get are the occasional spiders or more plentiful mosquitos.)
All this said, I figure if I can keep something alive that’s not bothering me/dangerous in any way, I’ll do it.
Okay, enough heavy talk. The drink wasn’t wasted, the bug flew off with a new awareness of the preciousness of life, and this morning I [del]woke up with the bubonic plague[/del] have suffered no ill consequences as far as I can see. And the juice really hit the spot! So thank you all for your input. 
- It’s not religion that keeps me from being able to kill bugs, as I’m agnostic leaning more on the atheist side than anything else. At the root of my problem is a ridiculously high level of empathy for living creatures, particularly those under my direct control. I just can’t shake the feeling that I have no right to exercise the power of life or death over something else; it seems so random and unfair. And yep, I’m a big fat hypocrite because I totally eat meat without a second thought. But like I said, it’s mostly the direct hands-on notion of having to do the job myself that ties me up in knots of anxiety. Maybe there’s also a karma element involved? If I save a bug maybe I won’t die randomly? I have no idea.