I have a number of columns separated by tabs but the spacing between them are of different sizes. Is there any way of separating them with tab of even size?

Thank you.

you can format the output with printf.

echo 'x y' | awk '{printf "%s%16s\n", $1, $2}'
x               y

Sorry for asking something which may be obvious but what do %s and %16s mean?
Thank you.

those are formatting specifiers of printf.
google printf for more information.
%s is for string. %16s means prints the $2 as a string of 16 characters (right justified).

Have a read of Using printf Statements for Fancier Printing.