What to do when reviewers want you to expand on introduction and discussion but you have space limitation? 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?
"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.