You're doing great!
I like the larger size, but the choice is yours.
Let me know if you'd like to try anything else, for instance:
Some people don't like the caption line, for instance, and it's possible to get rid of it.
Sometimes the sizes Flickr offers (you clearly know how to choose different sizes on Flickr) aren't the size you want. It's possible to use the board's software to make an image smaller and to customize the size.
Just let me know if you'd like to try anything else...and of course you can try text (highlight a word or two), and start clicking the buttons over the editing window, and then click "Preview" to see what you get (it shows up over the editing window).
When you like what you have, click "Post."
Have fun (and let us know if you have questions)!