Sample size...

have found numerous on-line calculators and have read through some forum postings but am now more confused than ever.

I have been asked to calculate the sample size needed for a study looking at cellular changes after a type of operation. Where do I start when I have no idea what the "population" is or no idea the extent of the change that is expected ("could be anywhere from 5 - 60% difference").

Any help or guidance?

Thanks in advance
You must first determine which statistical test you expect to use. The sample size calculation should be specific to that test. The size of the difference is a judgment call. Maybe enter 60% and see if the calculated sample size is reachable.