It's not full clear what you mean and which version/fork of jqGrid you use. Free jqGrid is the fork which I develop. It contains the feature for automatic resizing of columns on double-click between the column headers or direct by loading. See the wiki article for additional information.
If I correctly understand your problem then you should add cmTemplate: { autoResizable: true }
option to set autoResizable: true
property in every column and to add autoresizeOnLoad: true
option jqGrid to automatically resize all columns after loading the data.
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