As of now you have 25MB of upload space. Additionally, you cannot upload a single file that is larger than 1.5 MB. Upload space can be increased for some blogs, i.e. departmental, administrative, and group blogs.
If you are uploading large pdf files or images it may be preferable to use an outside service that you can integrate with Longwood Blogs, such as Flickr for your photos, YouTube for your videos, and/or DivShare for your files. This is probably the best practice, because if you ever want to move your blog to another service you will have persistent links to documents, videos, and images that you will not have to update for the blog when it is moved.