With the ATCA, unless you are imaging a visibility file with multiple configurations, you do not have a lot of spacings to play with, so you should do this rather carefully. If you taper too heavily, you will weight down too much data and there will be insufficient left to make a decent image. As well as applying a Gaussian taper, you also have control over how the visibilities are weighted through the uvwtfn adverb. You can select uniform or natural weighting. Uniform weighting gives slightly higher resolution and a better sidelobe response.
Natural weighting gives better sensitivity and is generally preferred for detection experiments.
The default is uniform weighting. The default of zero means that the normalization is determined from the number of ungridded points falling in each grid cell. These bigger boxes do what is called super-uniform weighting. They are sometimes useful for sparsely sampled data. Note that if the box was as big as the longest spacing, then you would have natural weighting again, as all the visibilities would be normalized by the same number.