While creating your project, you can import as many photos as you'd like, up to 1.5GB worth per session. In order to keep your file size down while creating your project, you can remove any unused or unwanted photos in one action.
Instead of removing all unused images, you can instead remove photos one by one. Simply hover you mouse over images that you want to delete, and you'll see a red circle in lower right of the image. Clicking that will remove that image from your project.
Note: If the image that you're attempting to remove has already been used in the project, you'll notice a green check mark in the upper left corner of the image. If you try to remove that image from the project, you'll see a warning indicating this, and to confirm if you really want to remove that image. If you do remove an image on accident and wish to put it back in the Media Tray, you can click on the Undo button.