I would like to encapsulate the grid into a box and give the box a padding and margin. The padding and margin don't seem to affect the width of the grid.

Then resizing the grid is complicated, because a setGridWidth function is needed to set the grid width and column widths right. Especially when the user has resized the columns. An extra complexity over /blog/?page_id=393/feature-request/resizable-jqgrid-my-solution-but-needs-improving/&value=width&type=1&include=1&search=1

Maybe I overlooked a functionality, but is there a possibility to make those widths relative?

You can use percentages instead of pixels to determine grid width and column widhts instead of pixels.

Percentages can be set with an accuracy of .1%. For example a grid of 1680 pixels wide can be set with 1.7 pixels accuracy (=.1%).

(What disadvantages comes with this change?)

Can you set the column widths and grid width to % instead of pixels?