I am currently working on a Customer Satisfaction Questionnaire. I have never done anything like this and I would really appreciate if somebody could please tell me how I can calculate the most reliable (but feasible) sample size for my survey. Apologies for my ignorance! :-)

As the supplier, you select the sampling plan and level you use based on the amount of defects you uncover in your day to day operations. There are levels of sampling in standards such as (ANSI/ASQC Z1.4-1993)or the old standby MIL-STD-105. I know your post is from Ireland, so I don't know the availability of these standards to you. You can always request them through your county's ISO Member representative. The ANSI would be my choice.
energy

Thanks a million for the info! I will check it out!

Paola - You may be looking more in the direction of sampling your customer base (what's a good number of surveys to send out at a time, how often and to whom). We have set up a program sampling 20 customers a month. They are chosen at random and once questioned, they are excluded from the selection base. Our questions are numerically scored by the customer from 1 to 10. Additional spaces are provided for comments. Once returned, the surveys are entered into a spreadsheet and average numerical values are charted for each month on each of the questions. They are then reviewed in total and compared with the National Customer Satisfaction Index. All returned surveys are analyzed by management.
Be sure the survey is sent to someone who can give you valid and objective answers. Good luck.
Dave

Dave,
You are absolutely right, I blew that one. I got hooked on the "Sampling" aspect without realizing what it was that was being sampled.
It must have been a short circuit.