Instead of explaining every issue, I am just going to edit what you wrote with correct sentences.
Hello, this is Tomm. I'm calling regarding the on going issue that you mentioned in the meeting this morning. I understand that the place that you found is not a large enough space to hold 40 people for the seminar, and that it can just accommodate 20. [How about] I suggest that we make two sessions for the seminar. There are many advantages to doing this. First of all, with a small group, they can more easily concentrate on the presentations. And afterward we will have more time for our Q and A session as well. This is my suggestion. If you have any questions, feel free to contact me anytime.