The effects of source size on correlation detection, including the actual and normalized peak intensity and correlation profile, are theoretically analyzed. Numerical calculation and experimental demonstration are made for both the coherent and incoherent circular sources with an annulus object as an example. It shows that while the coherent source should keep the same size in correlation and MSF fabricating steps, the incoherent source size may be much larger in order to increase the peak intensity without considerable degradation of correlation profile.