This article is from the Food Preserving FAQ, by Eric Decker email@example.com with numerous contributions by others.
Make sure you get one tall enough for your needs. _Putting Food By_ had a
very good piece about actually measuring the height of the waterbath canner
before you buy it. If you are going to be canning qts, you need to make
sure that you bring a quart jar with a two-piece lid on. Get the waterbath
canner out of the box (or look for one already out), put the jar in the rack,
and make sure you have at least 3" of clearance between the top of the jar
and the top of the canner. You want to be able to maintain a roiling boil
(full tilt, manly-man, no-holds-barred boil, not a girly simmer) of water
comfortably over the jars, with enough clearance to make sure that the
boiling water isn't going to boil over onto your stove and into your burner.
Also need to know your stove. If you have a gas stove, can use the waterbath
canners with ridges; if you have an electric stove, you should use flat
bottom canners. Also with electric stove, measure the size of your burner.
You'll get best results with a canner that overhangs the burner by less than
Lesson for the newcomer to canning: a ruler is the most important tool you'll
have. Use it liberally.