The campaigns are set to expand with the amount of content that you add to your newsletter. You can add in blocks, like you mentioned and then stat adding text/images to expand the whole newsletter further. When adding in blocks, in order to make sure they go where you want them to (like the end of your campaign) you can click and drag them into your newsletter or you can copy an existing block. Here are some steps on how to manage the blocks within your campaign.
If you need want some more help with this and want to talk it through, we have an award-winning support team that is ready to help you out! Here's how you can get in touch with them. For help from other Constant Contact users, posting here in the Community is a great place to go. Are there any other ideas or suggestions from others in the Community?