The mismatch between a screen and a sheet of paper
A laptop shows a guide at roughly 30 centimetres wide, backlit, at whatever zoom you happen to be at. A4 is 21 centimetres wide, unlit, and fixed. Somewhere between those two, a region that was comfortably readable becomes a shape with a 34 in it that nobody can see.
It gets worse than illegible numbers. A region needs to be wide enough for a brush to enter it, put paint down and leave without touching the neighbours. On paper with a size 4 round, that is somewhere around 4 millimetres across. Below that you are not painting the region, you are dabbing at it and hoping.
The screen gives you no warning about either, because on a screen you can always zoom in.
The arithmetic, once
You do not need to do this every time, but doing it once makes the rest intuitive.
Take the width you are printing at and divide by the number of regions across the widest part of the picture. A portrait 21 centimetres wide with roughly 40 regions across gives about 5 millimetres per region on average. That is workable. The same picture at 90 regions across gives 2.3 millimetres, which is not.
Averages hide the problem, though, because regions are never evenly sized. The large calm areas take most of the space and the small busy ones get what is left, so an average of 5 millimetres routinely contains hundreds of regions under 1 millimetre. The average is reassuring and wrong.
This is why detail and physical size are the same decision made twice. Choosing one without the other is what produces the unpaintable page.
What the readability check is for
Pacyra measures every region against the physical size you set, not against the screen. When a region has no room for a legible label at that size it is flagged, and when the count of flagged regions climbs the warning gets louder.
The right response to a lot of warnings is almost always to lower the detail rather than to raise the size. Raising the size fixes the arithmetic and gives you a picture you now have to find A3 paper for. Lowering the detail gives you a guide that fits the paper you own.
The check is also why setting the canvas or paper size before exporting matters more than it looks. Set at the end, it is a formality. Set at the beginning, it is the constraint every other decision gets measured against.
Two things that go wrong at the printer
Scaling to fit. Print dialogues default to shrinking a page slightly so nothing is clipped, which quietly undoes the physical sizing the guide was checked against. Print at 100 per cent and accept the margins.
Building small and enlarging later. A guide built for A4 and then blown up to A3 for a wall display has line weights designed for the smaller sheet, so it arrives looking thin and washed out. Build at the size you are going to print. It costs nothing to rebuild and the engine gives you the same picture back.
When the answer is not paper at all
Above about A3, printing stops being the sensible route. Poster tiles will get you there across several sheets, and they work, but the joins are a real thing you have to manage.
For a canvas, projecting the guide directly onto the surface removes the size ceiling altogether. The guide is thrown at whatever size the wall allows, lined up against the four real corners of the canvas, and checked against a scale bar before anything is drawn.