I THINK that Rhoda's bigger food causes just "small" growing and shrink soda's effect would be too big for it. But even if that's right, I'm sure she can get shrinking soda spell next.
True, I didn't think about that. (The shrinking soda popped into my head as soon as I finished the last panel, and I got excited ). It does seem like the side effect isn't a big change in size, or else Dan Rhoda might have phrased it differently.
I was thinking the change was along the lines of a proportional change. Like if you start with a bite size cookie, make it a regular size cookie with Rhoda's spell, you grow with the first bite/first few bites so that the rest is now bite size. This would mean you just need a couple liters or so of soda to make some can-sized shrinking soda. That would lead to big size changes fairly quickly, though. Maybe today's comic will bring more information about the side effect.