To the best of my knowledge, text files are only limited in size by the amount of space available on the drive where they're being stored. However, please note that the bigger the file the longer it takes to process, which is why I use hash tables and just load/save the hash tables as files when necessary. By using hash tables, you get the use of your RAM (which is where the table is stored) in addition to the hard drive storage (via windows memory management). Since hash tables reside in RAM the are a lot faster to access.
I ran a quick test on the access time comparing a text file with 25000 items in it, and a hash table with the same number of items. To read the 21,548th item (a randomly selected item) in the text file took 5.6 seconds. To get the same information from the hash table took 0.7 seconds.