I believe the correct flag is --ignore=box-sizing , so
csslint --ignore=box-sizing css/
You do not know how to install this in .csslintrc, but if you use the SublimeLinter-csshint package, you can set the flag in the settings of the SublimeLinter package.
You can also add this parameter to the CSS file itself, since csslint v0.9.10 is like this (do not add any spaces):
/*csslint box-sizing: false*/
Jannes
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