How do you decide the Sample Size for an online survey?
Sample Size for an online survey is not a tough task but making relevant is. The sampling decision and process in any research is dependent on the level of population representation one is trying to achieve in that research, and on the level of analysis (segment depth of analysis) one wishes to do. The more finely one wants to make the sample represent the population and its various segments, the higher the sample size one would need to achieve so.
Therefore, the sample size, at least in principle, is independent of whether it needs to be conducted offline or online. For the same population representation, the sample size will be the same whether the survey is being conducted offline or online.
It is usually for ease and cost (practical) reasons that sample sizes for the same research may vary under the two methods. In principle they don’t need to.