In which case the size is only to facilitate the contents. If large text is required even when there is nothing that text needs to contain, go large. If tiny text is needed, make it so. Sometimes it's necessary to establish a surrounding context, where the context isn't actually part of the literal puzzle answer, but there'd be no puzzle without the context. Without items 1 and 3, there'd be no reasonable reference to 2, but items 1 and 3 are not directly in the answer.
Substitution also comes into play here.
If one symbol or glyph can be substituted where another is expected, do so. Since the puzzles will show up in email which is typically on a white background, we use a light gray for the background of the puzzle image so that its boundaries are easily determined. You've been warned. You could say it, but you'd probably be wrong.