Eyepiece diameter for small focal length planetary viewing on a large dobsonian?
I would recommend a quality 1¼" 9 mm or 12 mm eyepiece as the optimal solution. 

You already have a 19 mm so something close to that such as 15 mm would bring small difference, so take a step or two down to a 9 mm or 12 mm Plössl lens.

While a 2" eyepiece usually offers better eye relief (from bigger lens) the cost is also substantially more, and if you compromise on quality by buying a cheaper eyepiece the results will be less than satisfactory - lack of clarity, distortions from imperfections in the glass, etc.

Extra magnification can also be obtained by using a Barlow, but you are also adding extra lenses to the optical path which will slightly reduce the amount of light you are getting at the eyepiece, but also any imperfections will be magnified.

Also, with Dobsonians (usually) not having any tracking mechanisms you will be manually tracking your object of interest, and extra magnification means that it will be moving more quickly across your field of view as well as making it a little more difficult to keep it smoothly in the eyepiece. These are also arguments in favour of a 2" eyepiece over a 1¼" eyepiece, but either way, good eyepieces can be kept and re-used when/if you upgrade your telescope.