Hey, for summer I got following setup:
F: 225/40/18 (8.5'' wheels)
R: 255/35/18 (9.0'' wheels)
Suspension lowered with tein high.

What's are the biggest winter tyres I can go for that would fit in? Should I go for bigger profile/ size or both?

Currently I got 16'' wheels (6.5 or 7.0 wide) but they're too small for the car and it sits even lower which is annoying - I have to be super carefull no to scratch the bumper. Actually, it is impossible to travel to the mountains for skiing and well I wouldnt either feel comfortable on the same tyres' size that I got for summer.

 

I used to run 16 snows on my 250 when I had the OEM front brakes.
Used a 205/55/16 on a 16x7 rim.
I would just use the 16's for winter and your summers for summer.

In reality with the proper size tire the car shouldn't sit any higher or lower than with the summers.
The summer tires you are running are the OEM size for an F Sport model, so a 205/55/16 on the 16 inch rims will keep the vehicle at the same height.
Ideally a narrow tire is better for winter as the goal is to cut through the snow, not ride on top of it.

 

in winter I use 225/45/18 on all wheels