Try to find the longest protein that is present in the SwissProt database.
Hint: Look under databases in the 'indexed fields'; perhaps the sequence length has been indexed!
MRS with query length>10000 gives ~10 hits; length>20000 gives ~3 hits;
length>30000 gives ~2 hits. The longest is titin with over 30.000 residues.
See also Titin in the Wikipedia.
The human titin is a bit longer than the mouse one...