How to pick a research question broad or narrow one? I am a PhD student of computer science ( senior year ). I have worked on problems given by my senior collegues in the past. Now I am at a stage where I have to pick a new research problem. I have tried to read as much as possible about particular topic, but still facing difficulty in picking a new problem. I have find out few research problem, but they seems to special cases. I am good with solving special case not a broad problem.

Question : Should I pick a research problem with broad or narrow one (special case )
Real broad/general problems are of course nice, because they apply to many situations. However, what students consider broad/general problems are often ill defined problems or problems that would require an unreasonable amount of time and resources to answer. So beware, that your broad problem is really a broad problem and a realistic problem.

After that is just a matter of finding the most interesting problem you can come up with, that is solvable with the time and resources you have available. (Remember to multiply your best estimate of the time it will take by two.) 

That is very broad advise, but we cannot give you more detailed help on that as we don't know your skill level, the resources you have, the time you have available, and most of us are not an expert in your field. Your advisor on the other hand (should) know all that. So come up with a couple of problems, list the pros and cons, your estimate of the time and resources needed to complete these, and ask your advisor for his or her opinion.