Does anyone know what the maximum length for a dryer vent can be? And is there a special cap for venting straight up through a roof?

I have not seen anyone vent up through a roof and I would not recommend it. No matter how good your filter is, there will be some lint expelled from the dryer. Not all of it will get out if it is sent up through the roof. Pretty soon it will clog your vent and possibly start a fire.
Bad news.