Large protein strings when doing BLAST?

How large is a "large" protein string normally when doing BLAST?

Will do searches for protein strings say consisting of 32KB of  
letters? Is there an upper limit? I suppose there is no real upper  
limit, but there might be a tendency for proteins to rarely go above  
a certain length... I am trying to design some hobbyist software in  
biology and these sort of answers would really help my design. Excuse  
my lack of biology background!

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