Generally speaking, you do whatever you want with your own webspace. But since you are at a tumblr blog, the etiquette there is (from what I’ve seen) to post smaller posts. It’s so easy to set up your own blog [so easy that it was done for you already :)] where you can post your bigger entries, then you can post a link on the tumblr blog (thus satisfying the “short entry” criteria) for those interested in the long read.
I host my own blog at myreallifename.com, thus I have fun posting whatever I feel like. Of course, I really blog for myself, I have no clue how many people look along nor do I really care to find out. Usually when I feel like focusing in depth on a topic, I will make a sub-blog dedicated just to that information so I can theme it and organize it distinctly. Setting up Wordpress (and thus installing a blog) is so easy to do, it pretty much installs itself! And for those who aren’t interested in doing the maintenance, Wordpress offers free blogs on their site as well.