This thesis aims to extend the existing knowledge and enhance the methodological quality of future stepped-wedge cluster randomised trial (SW-CRTs). A systematic review of published SW-CRTs shows that pre-trial sample sizes calculations display a poor standard of reporting, with little adherence to published guidelines. The methodological rigor is often substandard, with inappropriate methods often used to determine sample size. In SW-CRTs, it is assumed that the correlation between observations is independent of the timing of them. We test the validity of this assumption by outlining a method to estimate the within-period and inter-period correlation. A case study illustrates what these correlations may look like in practice. The impact of varying cluster size in a SW-CRT is then demonstrated by comparing a design with unequal cluster size to a design with equal cluster size. A simulation study provides evidence that the SW-CRT is affected less, on average, than a P-CRT by varying cluster size. However, the potential power in a SW-CRT with unequal cluster sizes is extremely variable. A practical method for estimated power in a SW-CRT with varying cluster size is then illustrated through a Stata function.