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The gridding stage requires at least Nyquist sampling of the uv plane to avoid the artifact known as aliasing. This sampling depends on the source size.
For the signal, the source size is limited by the primary beam, so that the Nyquist sampling in the uv plane is obtained with a size of the grid cells equals to half the size of the antenna diameter. (this is the smallest spatial frequency that the interferometer can be sensitive to, i.e. the natural resolution in the uv plane). In the image plane, this implies to make an image at least twice as large as the primary beam size (see Fourier transform property #2 in Appendix).
Unfortunately, the spatial frequencies of the noise are not bound: the noise increases at the edges of the produced image because of the noise aliasing and gridding correction.
Unless you have good reasons (such as a strong confusion source close to the primary beam), you should not choose too large an image size, since that would slow down imaging and deconvolution.