mouse-scroll-min-lines

mouse-scroll-min-lines is a variable defined in `mouse.el'.
Its value is

1

Documentation:
The minimum number of lines scrolled by dragging mouse out of window.
Moving the mouse out the top or bottom edge of the window begins
scrolling repeatedly. The number of lines scrolled per repetition
is normally equal to the number of lines beyond the window edge that
the mouse has moved. However, it always scrolls at least the number
of lines specified by this variable.

You can customize this variable.