My office is 1/2 of that size, but similar in that it's mostly glass. You're lucky 'cos in the summer months the sun will be north of you and away from that big south glass wall.

I had a 12K btu air con in my office and it wasn't nearly enough. I had to start it early in the morning to give it a chance to keep the office just mildly hot in the afternoon. This is with outside temps in the 90s.

So, you've got double the air con, and double the space and windows. Even with the solid north wall, you might consider the next larger size air con. And at least two of those heads.