How to make sqlplus output appear in one line?
It is not enough to force sqlplus not to wrap lines. It is also necessary to tell the viewer that you use to view the spool file not to wrap lines.
If your viewer is less then -S the option you have to use according to 
https://superuser.com/questions/272818/how-to-turn-off-word-wrap-in-less .
On Unix/Linux you can use head -1 output.txt to get the first line of a file and so check if this is as expected or you can use od -c output.txt|head 
to see where the line breaks are actually placed in your output file.

If you display LONG columns and their values contain line breaks then multiple lines will be printed for these column values and you cannot overrule this with sqlplus settings.



The following sqlplus commands maybe usefull:


SET LINESIZE linesize the length of the line. In most cases the maximum value for linesize is 32767.  You can find out your maximum value if you set LINESIZE to an invalid value and check the error message
so SET LINESIZE 0 may give SP2-0267: linesize option 0 out of range (1 through 32767)  (1)
SET TRIMSPOOL ON otherwise every line in the spoolfile is filled up with blanks until the linesize is reached. 
SET TRIMOUT ON otherwise every line in the output is filled up with blanks until the linesize is reached. 
SET WRAP OFF Truncates the line if its is longer then LINESIZE. This should not happen if linesize is large enough.    
SET TERMOUT OFF suppresses the printing of the results to the output. 
The lines are still written to the spool file. This may accelerate the exectution time of a statement a lot.
SET PAGESIZE 0 to set an infinite pagesize and avoid headings , titles and so on.
There are some other SET parameters concerning output (NUMWIDTH, NUMFORMAT, LONG, COLSEP)  and performance (ARRAYSIZE, LONGCHUNKSIZE).


You have to use the COLUMNcommand to format individual columns. 

E.g. column name format a30 will format the column name in the output to a maximum length of 30 characters. 

If you want the display size not be fix but it should be equal to the size of the actual value of a columns in a row then the only way I know is that you change the select clause of your statement to get the desired result and use the 
string concatenation operator || , e.g.

select emp_id||' '||first_name||' '||last_name
from emp;


A full description of all variable can be found in SQL*Plus User's Guide and Reference.

If you want to reuse some settings (or COLUMN definitions)  you can store them in a file and run this file when you need them again. You even can run this file automatically if you start sqlplus.

(1)  "How to Find the LINESIZE Maximum Value (It is System Dependent) (Doc ID 1547262.1)"