I think you are looking for changing the width of the actual items. The image size is somewhat actually smaller than the item width. So I rather suggest changing the item width instead of the image width.
On the Theme -> Isotopic layout panel, you are able to do that. The default width is 150px. If I may recommed, you should try a percentage value like 25% or 20% or 32%. Percentage values will stretch the items to better fit the content width. I only recently discovered that percentage values work better than exact pixels.Best,
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