Sample sizes in experiments can be a good indicator of a reliable study, the bigger the study the more robust the evidence. Though this isn't the only thing to be considered, there are others covered by our fact of the day such as self reported outcomes.
It is often useful to look at the sample size when considering if a study has been well-planned and that the methodology is sound. Of course, depending on what is being studied the idea of a sample size that is "good enough" can be a tricky one to pin down. There are mathematical models that can be used to figure out what a "representative" sample size needs to be.
We often see small sample sizes being used to justify the efficacy of certain treatments in the field of alternative medicine, and while small sample sizes can be used as an indicator for further study using larger sample sizes, care must be taken when using such small data sets to make wide conclusions.