Tiny mouse pointer within VM

[mr.burns]

Dear all,

I am quite new to virtual box and my problem is that I have a tiny mouse pointer if my mouse enters the virtual machine.
VirtualBox is up to date, I downloaded and installed today. My host OS is Windows 8.1 64 Bit. My guest OS is Windows 7 Professional 64 Bit.
My machine is a Lenovo IdeaPad Yoga 2 Pro 13 inch with Intel Core i7, 8GB RAM and 512 SSD. The screen resolution of the Lenovo Yoga is 3200 x 1800 Pixel, tis is the reason why the mouse pointer normally is so small.
But in Windows I changed my display setting to 250%, so the icons and letters appear bigger. This works but unfortunately not for the mouse pointer.
Changing mouse pointer size in control panel --> mouse does not have any effect.

How can I get my mouse pointer bigger now?

Thanks for any help!