[TheLastSuperman]

Responsive is the way to go if possible, otherwise set it in % values so it scales accordingly. You may have a good reason for wanting a set "fixed" width however I would not limit your users that way i.e. have your forum style completed using a fixed width BUT then copy it (or create a child style) so your members have a second option and set the new style to % values .

^ That way you're not limiting your members, if you have no mobile style installed then they load the site with tons of extra bandwidth being eaten up from their mobile data plan, if you have a mobile style installed then you can still get by with a fixed width as you have an alternative less data usage friendly version.