Shoot for something between 500 and 1500 words (If you want to do a piece that doesn’t fall within this range, consider running the concept by the editor for feedback.)
The blog crew is diverse, and you are expected to be who you are and to do what you do. Collectively we’ll cover a lot of bases. That said, the broad categories that your posts might fall into include news, reviews, translation, teaching, criticism, writing, tributes, obits, current MQR issue (and issues raised by it), lit world, art world, big world, etc.
Do your best to keep your first paragraph “excerptable,” and try to end it in such a way that readers will knock themselves out to click “continue reading,” or craft an appropriate excerpt for your post that achieves these ends.
Be yourself, and be “familiar.” You were invited to join the crew because the editor (and others) appreciate your voice/ experience/work. The editor is confident that you have something wonderful to contribute, and your tone should make readers feel as if they are having a conversation with a warm and acquaintance. Think epistolary and/or personal essay, and by all means avoid anything that approaches the distancing often characteristic of academic writing.
If in doubt, contact the editor, who will be happy to discuss proposals and/or take a quick look at preliminary drafts.