The Page Settings screen sets paper size & controls the placement of the grid on the page.
Adjusting margins also gives you a bit more control over the grid itself:
Add more rows & columns to a grid by reducing margins
Reduce number of grid rows & columns to a grid by increasing margins
To set paper in GridMaker or QuiltPaper — Choose US or International paper size at the pop-up on the main screen by tapping the setting icon.
To change paper size — Tap the top item to open a wheel where you can choose from standard paper sizes or set to a Custom Size. [A PDF with a very large grid can be printed, but you'll need access to a large format printer that can handle it.
To change margins — Tap each corresponding margin to adjust it's size.
The black line outlines the edges of the paper — you'll see the grid 'moving around' as you make adjustments. Dismiss the pop-up to easily see the new placement. Use pinch-and-zoom to adjust the image size so it fits on the screen, tap 'Page' button to Reset size.
The blue dotted outline marks the margins — sometimes there is a gap between the grid and the margin because GridMaker and QuiltPaper only show full-size rows & columns.