gosh I love a terra cotta flu
added to what the others have said...
Your 6.5 in x 10.5 is perfectly fine for that stove (or most any other for that matter). capping that flu is not important If you run your stove 24/7 during the winter (that would evaporate anything coming down), but if you don't than I would put a cap on that flu (coal ash being drizzled with rain water eats stove pipe so fast it would make your head spin), you should be very happy you have a terra cotta flu in place which is perfect for coal! there's some easy, fairly inexpensive caps that can clamp right to it that will do the job.
as with everyone else... Im steering clear of the baro/mpd recommendations (just roll with the manufactures recommendations or review the threads here about it), because that whole subject gets pretty hot..."no pun intended"