I would expect that it could work similarly to the method when selecting the range of cells with keyboard - SHIFT+Up/Down where the result is that selection is increased by +1 cell. So with SHIFT+PageUP/PageDown, the selection would be increased by +“x” cells. “x” means the number of cells that are visible on one screen, or as I would say “on one page” - depends on browser window size, display resolution, etc. Typically it is something like 24 rows on the screen, when the browser window is maximized, but can be less or more.
OR you (developers) could set it to a fixed number, something that makes sense, f.e. 10 or 20 or 25. Maybe 10 is best.
But I would prefer just whole page (number of rows displayed on the screen), if it is possible to implement.
This thing can make the selection process easier and faster (now the slowest one is by dragging a mouse, bit faster is by keyboard, incrementing by +1 row; if using PageUp/PageDown would be very fast).