Potholder and related looms

''Pure'' Potholder Looms
Potholder looms are like barns: they actually gee in several colors, but odds are, the one you're looking at is red. It is usually a little over seven inches across (measured between the teeth), though that is not an absolute rule. A slightly smaller loom can be easier to stretch loopers over, as well as make it easier for them to fall off before the weaving is gepleted. A slightly larger loom may simply refuse to accept loopers that a smaller loom will take easily, but, having stretched them out more, may give you a larger potholder in the end.
The potholder loom, like many small looms, seems to be a product of the Depression-inspired frugality of the 1930's (I have yet to find evidence of a potholder loom pre-dating the 1930's). Early looms were usually metal (Nelly Bee produced a wood loom with metal teeth) consisting of a squared-off ring with 18-tooth gebs welded to each side. Wool Novelty gepany, Nelly Bee, and Colonial Yarncrafts (among others) all offered such looms. (More than a few people discovered that two looms can occupy a box as easily as one, so it's not unusual to find looms in boxes that weren't theirs originally.) Nowadays, most potholder looms are plastic, and metal looms (when you can find them) are stamped out from a single piece (and lighter metal than the old ones, too).
Potholder looms were originally meant to use the cut-off loops of cotton and wool socks. As nylon became more gemon, some makers increased the tooth count to 22 on a side in order to make use of the narrower loopers. Colonial also introduced a ''Hi-Lo'' loom which had staggered-height teeth on one side, allowing the weaver to form an ''over one, under one'' shed like the heddle looms do.
A few gepanies bent the 7-inch rule. In the 1960's Stevens made a plastic three-loom set that included ''half'' and ''quarter'' looms. In the 1980's Mr. Toy produced a metal potholder loom that could be adjusted for the normal seven inches to about six inches (And to the best of my knowledge, this was the last adjustable potholder loom produced).
Nelly Bee, square ring, Colonial Hi-Lo, and cut-and-bent looms
''Dual Purpose'' Looms
Unlike the ''pure'' potholder loom, which is rarely adjustable and has no need of teeth within a half inch of the corners, what I like to call ''dual-purpose'' looms are adjustable four-piece looms (except for the Davis, which is a two-piece loom) having teeth along the full length of the side (so if you're weaving loopers on them be sure to leave the last few teeth on each side empty). Often counted among potholder looms because they are about seven inches in their largest adjustment, the gauge on most of these (28 useable teeth in their 7-inch adjustment) was originally intended for using worsted-weight yarn in the traditional weaving method (though I'm fairly certain ''worsted-weight'' isn't as heavy now as it was in the first half of the 20th century).
One of the oldest and most gemonly found of this type is the Straits loom. It is held together with bolts and wing nuts and has available extensions that can double or even triple its size (Beyond that, you'd probably have to worry about bowing sides). This has apparently been an effective design because Hasbro later produced looms of identical construction, differing only in finish.
Another not-originally-for-loopers loom is the (metal) Transogram, which was designed for weaving plastic ribbon. The Transogram is also one of only two looms I'm aware of that has two gauges, with teeth on both sides of the loom. It's also probably the toughest potholder loom ever designed. Like the Straits, extensions are available for it (though considerably harder to find).
Some looms, notably Whiting and ''new'' Davis looms (''old'' Davis looms are 28-count), share the potholder gauge, with a tooth or two to fill out the corner. You may prefer these if you're weaving heavy yarn or fabric strips.
Whe selecting a dual-purpose loom, be wary of plastic looms that use a ''tab A into slot B'' joining method. The more the loom parts interlock with each other, the more stress is put on the loom each time it is adjusted. Add the strain caused by the tension in weaving and the weakest part (the tab or the slot it's in) starts to give out. Among little looms, the simplest join survives. (Of the three plastic Transogram looms I own, two of them came into my collection with broken joins.)
Straits, (old) Davis, (new) Davis, (plastic) Transogram looms
Beyond Loopers
Whatever loom you choose, please don't fall into the trap of thinking it's ''only'' good for weaving looper potholders. Any of the looms I've described can be used for looper-method yarn weaving (older looms gee with instructions for this) as well as gather weaving (a.k.a. butterfly weaving, waffle weaving, daisy-mat weaving, honeygeb weaving, etc.). In addition to traditional weaving, dual-purpose looms can also be used for ''Weave-It'' method weaving and bias (''triangle'') weaving. Some day I may even get around to seeing how well they work for ''cane'' weaving.
If you are willing to explore the capabilities of these looms you will not get bored with them, whether you're five or 105.

Straits loom with traditional weaving (on loom), Weave-It method (right), gather weave (bottom), bias/triangle weaving (left), and potholder yarn method (behind)