I have just received a revise and resubmit for a manuscript. A couple of the reviewers want me to expand the intro and discussion sections but my manuscript is already at the word max. Should I ask the editor if the will approve a longer paper? Should I assume that if the editor didn't say anything that it's ok for me to lengthen the paper? How else can I handle this situation?
First and foremost, ask the editor. Sometimes space limits are hard, and sometimes they are soft, and you often can't tell the real story just by reading the journal's stated guidelines.
If the limits are soft, you don't really have a problem. Nevertheless, don't get verbose: the reviewer probable didn't just say "Give more introductory material," they probably pointed at specific types of shortcomings in your current material. Focused improvement of material may not actually take much length at all.
If the limits are hard, then begin by writing as though the limits were soft. I always find it best to start by writing the paper they way you want to write it, and then figuring out how best to reduce. If you start by worrying too much about length, then you are likely to cut the wrong parts, because you can't see the structure of the whole while you are writing. Once you've got a paper that's too long, then this question has some good advice about how to shrink it.
"Expand" doesn't necessarily mean add words. You can add material to your intro yet still condense the manuscript by making other edits somewhere else.
Sentences can often be restructured to save on word counts. I've managed to cut hundreds of words from papers simply by revising and editing. Sometimes you can eliminate words or phrases by using langauge that is more direct and concise. Do this right, and you can reduce your word count while still making your paper more forceful and substantive at the same time.
It might take some time and effort, but have the right mindset. Don't assume your paper is currently as concise and polished as it can be – assume there is room for improvement.