I try to keep a consistent font-size, yes. Unless my characters are raising their voices/shouting, or lowering their voices and whispering, all text stays the same size.
It makes for a more cohesive look to the comic page, as in my mind at least, I read larger letters as louder and smaller letters as quieter; if the size keeps shifting from panel to panel, I get the impression that the characters are TALking liKE THis alL THe tiME. Which is, you know, a bit odd.
I always, always make sure to include word-balloon-blobs in the very earliest thumbnailing stages, and it's the second thing I do once I start doing full-sized digital versions; first I layout the panels, and then I put the stick-figures/word-balloons in, to make sure there's enough space for everything, and that it isn't crowding anything important out.
I try to make sure that I place the bubbles in such a way that it a.) is easy to tell who is speaking, and in which order, and b.) lead the reader's eyes from panel to panel in the right order. I also try whenever I can to obscure as little as possible of the speaking character.
Paying attention to bubble-placement pays off! Placing a bubble between two characters can be used to silently imply a barrier or sense of separation between them, for example.
