The default size of campaign's Long Description window that shows in the affiliate panel is 450 x 150 pixels. Of course, the window is resizable by the affiliate.
The merchant can change the default size of that window easily:
Navigate in the merchant panel to Configuration > Affiliate panel > Menus & screens (tab) and there lick Edit next to the Campaigns page
In the next screen choose the Campaigns grid settings tab and there you can set the desired default size of Long Description window