How big is too big

I know this question may not have any hard rules but I am interested in any experiences, thoughts or WAGs. I have recently moved to a new company. This company loves to write complicated long stored procedures. One such stored procedure is about 10,000 lines long. Needless to say the SP slices, dices and munches 7 ways to Sunday it also runs like a sick dog. My experience has always been to keep a SP simple. I try to keep them to no more than 100 lines but I have written some that were longer but none over 500 lines. I don't believe you can get good performance from a SP that is 10,000 lines longs. I am open to all opinions on this subject, hard facts would be better but I am easy. Thanks RAW