I think it’s the nature of a complex toolbox, there are always corners that get less light. The fact this issue I unclear doesn’t invalidate your query but suggests it mostly 'just works. Yet clearly, tight up to the point … where it doesn’t! I.e, your question.
I think there are possibly two interlinked factors at play: general margin width and embedded text styles. I’m not sure of the presumed norm for Cmd+4. Text-only more. Here, on one of my pair of 24" monitors, my main working space, the text re-flows across the whole window, though clearly this is different from, say, the experience on a 13" or 14" laptop scree. Indeed, a user might assume the text width might expend to a certain (‘comfortable’) width and no more. The uncertainty of that indicates this is perhaps an element of design yet to be fully settled.
For users who go back to the ‘old’, i.e. pre-v6 app design, some base assumptions have changed. The old model allowed the text size (i.e. ‘print’ size’) to be altered separately from the in-app viewed size (a magnification factor). Whatever the backstory of that design, it didn’t carry through to the v6 design where the design intent (I think!) is you just set the size of the text you want—as rich text—and go with that.
So whilst margins are internal magic it seems, you do have these controls over text:
$LineSpacing](LineSpacing). This is the ‘leading’ between lines of (soft-wrapped) text. The doc level default (set at the System Inspector) can be set per selected note(s) using the Text Inspector
That setting is discrete from - if needs be… $ParagraphSpacing’. this controls the spacing between paragraphs. If you prefer, where a hard line break occurs in $Text. It too can be set via the Text Inspector.
Text font ($TextFont), size ($TextFontSize), colour ($TextColor) have system attributes. They can also be customised at the System Inspector, Format menu’s sub-menus. The $Text colour can be altered sing the Text Inspector.
If any note looks wrong, as noted above, select all and use either/both the Format ▸ Style menu’s ‘Standard Font’ or Reset Margin’ options. The former also has a shortcut: ⌃⌥⌘T.
So controls for just about everything except what you want—margins. 