Room to leave for a balloon
The text is set in a block about five to six words wide at the recommended size for the trim (condensed comic caps advance about half an em per glyph), then an ellipse 1.25 times the block on each axis with a little padding contains it. Over about 35 words a balloon becomes a wall; split it. The share of the page is a sanity check when composing: if one balloon takes a big share of a small panel, cut the text or split it.