I disagree with Kiko's view that narrow pitches encourage less football. The fact that there is less space to just whack the ball into for people to run on to means that it encourages people to actually play football instead.
Incidently, I think Arsenal's is one of the narrowest and they play more football than any other team in England so that backs it up.
And also incidently, their pitch has a special dispensation from
UEFA which allows them to play in european matches even though their pitch is smaller than the standard set for these competitons. This is in recognition of the fact that nothing can be done to overcome the problem.
The pitch at their new stadium will have to be bigger. Will this cause them a problem? Given that they have to play half their matches away anyway, I'd say no.