How do I know which size and type of rim strip to buy?
There are a couple of types of rim tape available:


Cloth - This has a glue backing and is very long lasting.  If you need to replace a spoke nipple, depending on the age of the tape, you will most probably have to also replace the tape since it will no longer stick to the rim.  
Rubber - This is continuous loop of rubber (stretchy) and the stretch is what holds the tape in place.  As it ages there is a tendency for the area around the valve stem hole to break.  The rubber can also perish or become brittle with age.  
Plastic - This is a slightly stretchy plastic in a continuous loop that can be slipped over the outer rim.  I found that the tapes have issues as they age with cracking, particularly when you are using high pressure tyres.  You can get little splits in the tape just above the holes of the rim. 


My personal favourite is the cloth tape, over time they have been the most dependable.

It is very important to get the right width of rim tape.  If the tape is too narrow you can get the tube being pinched into the rim hole, or cut by the hole.  If the tape is too wide then you can get the tyre being lifted of the rim when at high pressure (not instantaneous and leads to a huge bang).

I know you can get both the cloth and rubber tapes in various widths for the various wheel sizes, so shop around to get the right one for your wheels.