I’ve just used the following command:
SET LIN[ESIZE] 200
(from http://ss64.com/ora/syntax-sqlplus-set.html).
EDIT: For clarity, valid commands are SET LIN 200 or SET LINESIZE 200.
This works fine, but you have to ensure your console window is wide enough. If you’re using SQL Plus direct from MS Windows Command Prompt, the console window will automatically wrap the line at whatever the “Screen Buffer Size Width” property is set to, regardless of any SQL Plus LINESIZE specification.
As suggested by @simplyharsh, you can also configure individual columns to display set widths, using COLUMN col_name FORMAT Ax (where x is the desired length, in characters) – this is useful if you have one or two extra large columns and you just wish to show a summary of their values in the console screen.