Size and Length
Most standard aquarium airline tubing is all the same width (standard airline tubing for most all air pumps is 3/16" diameter). When it comes to length, I would probably buy the longest length available in the desired type. Airline tubing can be easily cut at any length you need using a pair of scissors, and a year or so from now you'll possibly be wanting/needing to replace your old tubing. If you want an approximate length then I'd suggest doing the measurements, but I'd tack on an extra 2 feet to that length as the ends of tubing tend to discolor a bit over time (I like to trim mine off when it does that), more so if you're buying clear tubing.
Texture/Type and Color
The typical colors for most airline tubing sold at local petstores is typically clear, blue, or black (although you can probably find a variety of other colors available online).
Most flexible airline tubing is either a smooth matte silicone type or a slippery shiny rubber type, and their is also a hard plastic pipe type, often refered to as "rigid" (similar to the shiny rubber type but not flexible). The silicone type is a bit more soft, flexible, and easier to work with but it can harbor a bit more buildup and bacteria since it has a slight texture to it. The rubber type is slightly less flexible than the silicone type, and can be a little harder to work with but its slick shiny surface is less likely to harbor buildup and bacteria. Less buildup in your airline tubing will also allow air to flow through the tubing at its optimal performance.
The silicone type is generally matte in texture, not shiny like the rubber type so, if you don't want any glare while possibly doing some flash photography then this one may be the way to go... although the shiny rubber type in my opinion looks classier and cleaner when viewing an aquarium in general. Often times I have noticed silicone airline tubing making claims that the tubing does not harden over time, I find that to be very false... unless of course you are changing your silicone tubing every so often. Both types of tubing can become hard over time and it doesn't matter if you have very hard water or very soft water.
|Clear rubber air line tubing comparison|
Not all brands of airline tubing are created equal. If you found two different brands of clear shiny rubber airline tubing (as seen in the above image) you may notice they both have a different amount of clarity (meaning one brand of tubing looks more clear, or even better quality than the other) as the PETCO brand is more clear than the TopFin brand, although the PETCO one is about twice the cost (although still relatively cheap in my opinion). You can also use all these different types of tubing together (such as silicone with non-silicone) and attach them with things like air check valves, connectors, gang valves, etc.
You should replace airline tubing if it gets damaged, discolored, dirty inside tubing, etc.
If you have solid colored airline tubing it may be hard to visually tell what's going on inside your tubing. For solid colored silicone tubing you may be able to tell when it needs replaced or trimmed by examining the cut ends of the tubing by squeezing these ends to see if it feels harder than the center areas of the tubing, if so then it's probably time to replace or trim off the hard areas of tubing as these areas could have bacterial growth and/or some type of debris blockage, etc. It could also mean that you've had or have some water or condensation built up or accumulating in the tube over a period time, so be sure you have an air check valve and that it's working properly, as doing so will help protect your air pump from weakening or breaking overtime due to water damage.
My Current Top Pick and Advice
I really do like and currently use this clear rubber type PETCO tubing on all my tanks and I highly recommended it in generally, especially if you have no background or a background other than black on your aquarium, and this particular PETCO tubing is also kink resistant (which is beyond great to find for this type of tubing). If you do have a black background than I recommend trying a black silicone type of airline tubing for the tubing going inside of your tank (if you want it looking less noticeable against a black background anyway) and then use the PETCO clear rubber type airline for outside the tank. The other great thing about clear airline tubing is that you can better view when your tubing needs to be replaced and to insure your air check valve is working properly and not allowing water to travel down into your air pump (which could ultimately slow and/or damage your air pump).