Help me to change the size of announcement.

my forum is http://onepiecevietnam.forumvi.com/
ver: phpbb3
I'd like to use the announcement system of forumotion basic form very much but...
Is there anyway to make it in shape but not full of the width screen like in the photo below


I wish to make the announcement banner just fix in 750 x 240 (wxh)
I don't know which part of the code I can edit to fix the announcement.
please help

Hi, there.

Can you paste that code on here please?

The problem is I don't know which part of the code is (in CSS or template?) where I can edit the announcement.
T_T which code do you want me to paste? in the picture is just the announcement basic when I turn on that system of admin panel.

Oh... my bad. I misread. it. The code would be in your announcements.