ORA-01555: snapshot too old: rollback segment number 201 with name "_SYSSMU201_1036634350$" too smal

Hi I'm having trouble tracking and planning on what size to assign my undo segment size. And i think i have alot of contention/waits in my undo segments:

SQL> select name, waits from v$rollstat s, v$rollname n where s.usn=n.usn;

NAME                                WAITS

------------------------------ ----------

SYSTEM                                  1

_SYSSMU11_3095925781$                  23

_SYSSMU12_4080339446$                  17

_SYSSMU13_381915210$                   22

_SYSSMU14_1293206807$                  14

_SYSSMU15_3207856038$                  16

_SYSSMU16_1448921938$                  20

_SYSSMU17_1794958003$                  14

_SYSSMU18_3047225948$                  18

_SYSSMU19_3673066933$                  62

_SYSSMU20_935038634$                  227

SQL> select * from v$waitstat;

CLASS                   COUNT       TIME     CON_ID

------------------ ---------- ---------- ----------

data block             148607       3188          0

sort block                  0          0          0

save undo block             0          0          0

segment header            869         39          0

save undo header            0          0          0

free list                   4          1          0

extent map                  0          0          0

1st level bmb          176668       1065          0

2nd level bmb              41          0          0

3rd level bmb              17          1          0

bitmap block                0          0          0