Referees ask for more content but this lengthens the paper dramatically So, my situation is as follows: I submitted a paper to a journal and continued to work on that line of research, specifically on the open problems I mentioned in the submitted paper.

Now, in the received reviews the referees call for more content in the form of addressing one of the before mentioned open problems in particular. I am fine with adding more content, especially since I have already done the work.

However, the solution to the problem fills an additional 25+ pages, an amount which would pad the paper to a very long length. I am not sure of how to proceed as I am quite convinced that merging the two projects would be too long for the journal.

I don’t know if I should 


merge the projects and hope for it not being too long,
disagree with the referees opinion in my revision and state the situation,
contact someone and get information before conducting the revisions?


Maybe as a more general question, how is it in general perceived to get a large lengthening in return to a call for more content?
First, share your concerns with the editors. The reviewers may not have realized the extent of the work. If they agree that a long paper is fine, then write the long paper. If they don't want to do that then consider breaking the work into two papers with one referencing the other. You could even submit them simultaneously to the same journal. 

But it would probably be worth checking that with the editors also. I'll guess they would consider that to be the better solution, but it is their call.