Unequaled front rear tires

Steering and handling will be affected, this is not something that is recommended and it may be contrary to your local Road Authority safety certificate requirements.

As MPH says it will cause a lot of handling issues. Plus the different sizes on right/left will cause accelerated wear in the differential. Plus if it has ABS I would expect the ABS light to come on and quite possibly activate most of the time as the rotational speed would be different between the tire sizes.

If you have one tire a different size than the other side on the front, it will make you seem to always be going around a turn, even if slightly and have to continue over steering in the opposite direction in order to keep going straight. It will also throw off the front end camber and toe on the side that is different. When the rear is either raised or lowered by putting different than OEM sizes or the size that was on there it will change the rear ride height and that will throw off the caster on the front wheels making it easier or harder to steer. Not to mention different sizes on the front will make the trans-axle to appear to be constantly going around a turn and could possibly overheat it.