I live inSherwood Oregon and would like to know when and how short I should...

Thank you for your question. You can find an answer, as well as other helpful advice, in the following Extension article, under thr ‘trailing blackberries’ section: http://smallfarms.oregonstate.edu/sites/default/files/small_farms_pruning_berries_9-7-13_strik.pdf Hope this is of assistance. Good luck!