Increasing the width of the popup makes it show more assets on the
screen and reduces the need to scroll in bigger asset libraries. Plus,
in a follow-up commit the size of previews will be increased so that
there's more space to display the full name. Makes sense to increase the
popup size together with that, so a similar amount of assets can remain
visible still.
Increasing the size means it's more likely to overflow the window, so
this also makes sure the popup is clamped nicely by the window size.
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