The machine that makes up the PRNG of the RC4 cipher has (256!) * (256^2) total states, which is about 2^1,700. Consequently, it's suitable for enciphering any reasonable quantity of data.
Nominally, RC4 accepts a key size of up to 2,048 bits. However, as noted above, there are only 1,700 bits worth of total states in the machine. Recently, cryptanalysis of the cipher has shown that it can (in theory) be broken with a work factor of around 600 bits. Thus, it's quite reasonable to use with 128-bit keys.
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This was first published in April 2003