As a disclaimer, I have not been in this exact situation: in my field (mathematics) there are few conference papers, and the number of journals with strict length requirements is small enough that I have never reviewed a paper which violated the requirements. Nevertheless:
In my experience it is customary in academic and professional contexts that if you submit something that does not adhere to the rules of submission, then you should expect that your submission will be rejected for that reason alone. Now it may be the case that rather than definitively rejecting your submission you are told, "Hey, please fix X so that your submission can be considered"...and it may not. There is much talk of grant applications that are rejected because something in the fine print of the submission rules was not followed.
In your case, you have noticed that one of the submission requirements has been violated, apparently in a nontrivial way ("by more than 10%"). In my opinion your clearest ethical obligation is to convey this knowledge to the editors. It is really unfair if the requirement gets completely ignored and the paper gets published anyway whereas some other authors are either getting dinged for not following the same rules or are working much harder (and perhaps, trading on the quality of their paper) in order to follow them. So I think your first step should be to point this out to the editors.
If you like, you can convey your willingness to look at a new version in a timely manner. You could even say that you are reading the version that you already have and are willing to work on a report under the assumption that the authors will later submit a version which is essentially the same but meets the length requirements. But I think that's about as far as you can go. If the submission really is permanently rejected based on the length then the authors will resubmit to another conference/journal and they'll get their feedback at that time (possibly even from you!). Viewing the fact that you received the paper and are not at this time writing a report on it to the authors as some kind of disservice to them is probably the wrong way to view it: this is really part of the usual business of academic refereeing.
Added: As long as you point out the failure to meet the length requirement, I certainly see no ethical problem with passing on whatever evaluation of the paper you want. In fact, when communicating with the editors if you think that the paper is otherwise very strong then it would be useful to say that. However, it seems to me that it is possible that the editors might decide that since the length requirements have been violated the paper will be rejected and the authors will not receive a referee report. Thus your careful comments are not guaranteed to be conveyed to the authors. This seems like a good argument for checking in with the editors before writing a full-blown report.