at what value does SQLPLAN size go from being OK to being too big & why that value?
how big is big enough?
BTW, you have 1 record for every 12.5 seconds; which is a lower sample rate than AWR.
Ivan-I can't think of any issues at the moment except that you would run out of the space. May be others would be able to give a more meaningful insight. But in the meanwhile, did you try to see that might be some baselines can be removed?
the SPM is undoubtedly indexed on signature and/or sqlid (haven't checked but you can ) . a btree search is O(log(n)) so the number of rows is not likely an issue.