Commands based on the USEARCH 
algorithm, including usearch_global 
and usearch_local,
cluster_fast and
cluster_smallmem,, do not have built-in 
limits on sequence length. However, the word-counting heuristics and global 
alignments (if applicable) made by these commands tend to be ineffective or 
computationally expensive when the sequence length exceeds that of a typical 
gene, say somewhere around 50,000 letters. If you have a situation where you 
feel that one of these commands would be useful with longer sequences, please
email me to discuss.