Minimal record length is on 32-bit platform 32 bytes. On 64-bit architecture minimal record length is 64 bytes.
First limiting factor of record length is block size of each database file, default block size is 1MiB, but can be easily extended via file's paramater See Block size. For aditional informations about block size vs. record size see See Data block. Secondary limiting influence of record length is space reserved for represantation of variable part of record - currently 2^24 - 16MiB. The third limit is space for represantation of static part of record - currently 2^31 - 2GiB.
No record ( excluding BLOB part ) should be longer than the size of the data block ( default 1 MB ), but due to the design of interval tree average1, the record shouldn't be longer than 1 MB / 9 = ~ 110 KB See Interval tree.
Except previous few there are no arbitrary limits other than underlying file-system size and the sizeof physical volumes.
[1] average record means in no case each record, and you surely fall into data disaster, if you frequently exceed 110 KB limit per record - in the case of 1MiB block