Think carefully about the pros and cons for your company.
There is no minimum size that dictates whether a pension scheme is worthwhile. However, the type of scheme which is appropriate may well depend on the number and type of members (directors, employees, part-timers) together with the total contribution levels. It's worth going back to basics and asking why you want a pension scheme in the first place. Is it to offer tax planning for senior executives, to aid recruitment and retention of staff, to meet the legal terms of a take-over, or to meet your desire to provide employees with a decent pension?