The V$SGA_TARGET_ADVICE view provides information that helps us in deciding optimal value for  SGA_TARGET.
MMON background process gather statistics about sga_target usage and update the V$SGA_TARGET_ADVICE view.

Check sga target advisory is enable?

--- DB_CACHE_ADVICE should be ON

SQL> show parameter db_cache_advice

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- ------------------
db_cache_advice                      string                           ON

-- STATISTICS_LEVEL should be TYPICAL/ALL.

SQL> show parameter statistics_level

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- --------------------------
statistics_level                     string                           TYPICAL

SGA ADVISORY REPORT:

SQL> select * from v$sga_target_advice order by sga_size;

  SGA_SIZE SGA_SIZE_FACTOR ESTD_DB_TIME ESTD_DB_TIME_FACTOR ESTD_PHYSICAL_READS ESTD_BUFFER_CACHE_SIZE ESTD_SHARED_POOL_SIZE  
---------- --------------- ------------ ------------------- ------------------- ---------------------- --------------------- -
      6400           .0625     18334401               .9244           694123290                  78848                 22784  
     12800            .125     18334401               .9244           694123290                  78848                 22784  
     19200           .1875     18334401               .9244           694123290                  78848                 22784  
     25600             .25     18334401               .9244           694123290                  78848                 22784  
     32000           .3125     44322680              2.2347          2.1805E+10                   7168                 20224  
     38400            .375     34798471              1.7545          2.1805E+10                   7168                 26624  
     44800           .4375     28646014              1.4443          1.3125E+10                  14336                 24576  
     51200              .5     24847833              1.2528          7767170203                  21504                 24576  
     57600           .5625     22975519              1.1584          5125545198                  28672                 24576  
     64000            .625     21785489              1.0984          3448265680                  35840                 24576  
     70400           .6875     21006019              1.0591          2348774389                  43008                 24576  
     76800             .75     20528023               1.035          1674433612                  50176                 24576  
     83200           .8125     20226549              1.0198          1248380737                  57344                 22784  
     89600            .875     20028211              1.0098           968996113                  64512                 22784  
     96000           .9375     19899291              1.0033           786233451                  72960                 22272  
    102400               1     19833839                   1           694123290                  78848                 22784  -->> CURRENT SETTING
    108800          1.0625     19780288               .9973           619227387                  86016                 22784  
    115200           1.125     19738637               .9952           560921031                  93184                 22528  
    121600          1.1875     19704919               .9935           513165348                 100352                 22272  
    128000            1.25     19683102               .9924           479778018                 107520                 21760  
    134400          1.3125     19663268               .9914           452846034                 114688                 21504  
    140800           1.375     19647401               .9906           429592904                 121856                 20992  
    147200          1.4375     19633517               .9899           409393916                 129024                 20736  
    153600             1.5     19625584               .9895           394123204                 136192                 20480  
    160000          1.5625     19619634               .9892           394123204                 136192                 26624  
    166400           1.625     19609717               .9887           380310151                 143360                 24576  
    172800          1.6875     19605750               .9885           380310151                 143360                 32768  
    179200            1.75     19601783               .9883           380310151                 143360                 38912  
    185600          1.8125     19599800               .9882           380310151                 143360                 40960  
    192000           1.875     19599800               .9882           380310151                 143360                 40960  
    198400          1.9375     19599800               .9882           380310151                 143360                 40960  
    204800               2     19599800               .9882           380310151                 150528                 40960  

  SGA_SIZE SGA_SIZE_FACTOR ESTD_DB_TIME ESTD_DB_TIME_FACTOR ESTD_PHYSICAL_READS ESTD_BUFFER_CACHE_SIZE ESTD_SHARED_POOL_SIZE  
---------- --------------- ------------ ------------------- ------------------- ---------------------- --------------------- -
    102400               1     19833839                   1           694123290                  78848                 22784  -->> CURRENT SETTING
    153600             1.5     19625584               .9895           394123204                 136192                 20480  

Now if we see the estimated stats for size factor 1.5(i.e with sga_size 153600M), then there is an decrease in est_phyiscal_read from 694123290 to 394123204 , But no significat change in est_db_time. So By considering the estimated improvement in physical read, we can take suggest to increases the sga_target from 102400M to 153600M.