Lensless ghost diffraction is a kind of correlated imaging technique that produces the diffraction pattern of the object on the reference path by measuring the correlation of intensity fluctuation between the test and reference paths, in which the test path contains the object. The effects of the detector size and defocusing length on lensless ghost diffraction imaging with pseudo-thermal light are investigated experimentally. Using a four-slit object in our experiments, how the detector size and defocusing length change the patterns of lensless ghost diffraction is quantitatively studied. The experimental results are well interpreted with the numerical simulations. The image correlation is used to quantitatively analyze the differences between the experimental patterns and the idealized ghost diffraction images. It is found the diffraction quality is decreased with the appearance of the finite size of the detector and the defocusing length.