The material stainless steel is use because of its resistance to corrosion. There are many different types of stainless steel tubing used for a variety of applications. Two common categories of stainless steel tubing are standard fractional tubing and standard hypodermic tubing. They are categorized based on the size diameter of the tube.
Fractional tubing comes in sizes with the outer diameter being in fractions of an inch. For example: 1/16, 3/32, 1/8, 5/32, 3/16, 1/4, 5/16, 3/8, 7/16, 1/2 of an inch and so on.
For each tube of a particular outer diameter, you have various standard wall thickness giving you various inside diameter for your tube.
The size for hypodermic tubing are in "gage sizes". For example, a gage size of 6 will have a larger diameter than a gage size of 27. There are tables that tell you the specifications of the inner diameter and outer diameter for a particular gage size.
In the event that you need precise tubing diameters, you also have specifications for the tolerances of the diameter. Besides selecting the size, other considerations when selecting a tube is the "temper" ("full hard" for example) and the method of drawing using in manufacturing the tube. These selections will determine the price of the tube. The price is typically priced on a per feet basis.
You may need stainless steel tubing of a particular size that is non-standard. In that case, there are tubing manufacturers that will custom size and shape your tubes for you.