(Most of the material I read long ago in Ravivarma's വൃത്തശാസ്ത്രം and elsewhere. Now, writing from memory, so there may be some inaccuracies.)
Obviously, the shortest meter must be of one letter. Strange as it may seem, indeed there are names for such meters.
One with a single laghu is named ഖഗ. Its ലക്ഷണം is:
ലThe one with a single guru is ശ്രീ. Its ലക്ഷണം is:
ഗംI also remember one two-letter meter: ശിവം. Its ലക്ഷണം is:
ഇരുInteresting, but one might wonder, were there really poems made in these meters?
I don't know. I suspect not many, if at all. I believe that these were defined for the sake of completeness. (It is a very interesting detour that the വൃത്തശാസ്ത്രം has very deep binary mathematics embedded in it. For example, it talks about determining the number of വൃത്തം in a given ഛന്ദസ്സ് - which is equivalent to evaluating 2^n.)
The ശാസ്ത്രം also indicates that meters with less than five letters (five, if I remember correctly, may be it is six; someone please correct me), were intended for the devas. This might be a reference to Sanskrit. The next set of meters are for men and the last set are for asuras. Perhaps devas had richer vocabulary enabling them to communicate in terse, one or two letter lines!
One can find nursery rhymes in five letter meters. E.g.:
ക ഖ ഗ ഘ ങാ...A six-lettered meter I know is തനുമധ്യ. It is two guru's, two laghus and two gurus. Literally, the name means "thin in the middle"; figuratively, it means beautiful woman. Unfortunately, I only remember the first and last lines of the shlokam:
ക ഖ ഗ ഘ ങാ...
എനിക്കും താ ചേട്ടാ
Technically, this doesn't exactly fit the വൃത്തം (e.g. the first line is ഗഗഗലഗ, instead of ഗഗലലഗഗ, but I guess if you sing it appropriately it fits in).
P.S. A ശ്ലോകം with more than 24 letters in a line are called ദണ്ടകം. There are lots of കഥകളിപ്പദങ്ങള് in ദണ്ടകം format.