Actually there is, this is one of the most interesting metrics. It doesn't have to be precise though, rounding to nearest 50px would be more than enough. I would argue that height and aspect ratio should be collected too. (I didn't downvote you FWIW)
>>Drdrdr+(OP)
Could you round to buckets as well - take the 10 (say) most common screen sizes, and round users to the nearest? That way users with odd screen sizes aren't identified.