calibrating a Lidl wireless speedo

Hi
I've lost the instructions for my speedo and dont know how to calibrate it
for wheel size. If anyone can point me in the direction of some downloadable
instruction or aven tell me what my 105 wheels with conti 3000s should be, I
would appreciate it.

PS The unit is the small one that Lidl do with a temp sensor and a thin
lightmeter-looking-thing along the top...I dont even know what this does!!

Bev