I'm pretty sure there is no way to actually increase the cursor size, however it is possible to create an addon that adds a bigger cursor image at the current cursor position through an OnUpdate function combined with the GetCursorPosition() API
http://vanilla-wow.wikia.com/wiki/API_GetCursorPositionhowever this will feel clunky at lower FPS as the game cursor usually is not depending on the game FPS while OnUpdate functions only update each frame
might look into this later