Oracle Error: the sga requires more space than was allocated for it
Before you start make a copy of the files you change (the spfile) 
Check where your spfile is located, on Linux this is $ORACLE_HOME/dbs/spfile${ORACLE_SID].ora. If this does not exist check the content of $ORACLE_HOME/dbs/init${ORACLE_SID].ora. There the spfile parameter may be set which defines the path of the spfile. For Windows look in the %ORACLE_HOME%\database directory instead of.
Open sqlplus and create a pfile from the spfile create pfile='&lt;auxiliarypfile&gt;' from spfile='&lt;spfilepath&gt;'. Do not overwite an existing pfile but write the file in an arbitrary directory.
Edit the pfile &lt;auxiliarypfile&gt; created in step 3 with a text editor like notepad.
Open sqlplus and overwrite the spfile by a new created spfile create spfile='&lt;spfilepath&gt;' from pfile='&lt;auxiliarypfile&gt;'