Winter tires should be changed out when the tread has been worn down to around 1.5mm to 1.6mm in thickness, after they have been worn down more they are considered worn out. To measure the tread of ones tires one should use a tire gauge which can be bought at websites such as amazon.
Tires are made for the conditions that they are driven in. Therefore if you live in an area that has cold winters, that is it snows for a large portion of the winter, you probably should purchase winter tires. If you are living in Florida for example winter tires would not be required and year round tires would be sufficient.
You at least want the drive wheels to be winter for traction. For the best results all 4 should be.
In the winter, tires tend to loose capacity and shrink. Therefore making the air level decrease.
No need to change sizes with winter specialty tires. If your vehicle came with p235/75r15 tires, you can install the same size in a winter tire. Congrats on selecting a superior traction tire for safe winter driving, I suggest looking into the Nokian line of winter specialty tires if they are available in your area.
To order tires online you should know the make, year and model of your car and the size of your tires. You should also know what brand of tires you are interested in and if you require winter tires or all season.
For snow tires you should first make sure the tires have the M/S branding for mud and snow which rates them as winter tires. Cost effective brands include Bridgestone Blizzak and BF Goodrich Winter Slaloms.
Trelleborg and Vee Rubber are a couple of the many manufacturer's that make winter motorcycle tires. Trelleborg tires are generally studded, so check to make sure studded tires are legal in your state. There are other types of winter motorcycle tires available, however. They should be able to point you in the right direction at your local bike shop.
I use winter tires in winter because it often snows here. When it snows I stay on the road with good traction and steering. (as opposed to all the idiots I encounter sliding on hills and unable to stop at intersections )
First off you should not install tires that are not the correct size. But if you do, make sure the tires on each axle are both the same size. Both front tires same size, and both rear tires same size.
The type of style for wheels and tires depends on what you want to use it for. For summer, tires should be more efficient for the road, but for winter, there should be more traction for ice and snow
If you are driving in the area where the road is slippery to use winter tires.
Water is way too heavy to put in tires, plus in the winter it will settle and freeze so your tires will be off balance. Also about the winter part, if you completely filled your tires with water and let the water freeze, the tires would explode, since water expands when it freezes.