Taken from their help page......
Is there a limit or maximum to how big my files can be?
Files uploaded to Dropbox via the desktop application or mobile apps have no file size limit.
Files uploaded through the website (by pressing the upload button) have a 300 MB cap. In other words, each file you upload through the website must be 300 MB or less.
All files uploaded to your Dropbox must be smaller than the size of your Dropbox account's storage quota. For example, if your account has a storage quota of 2 GB, you can upload one 2 GB file or many files that all add up to 2 GB. If you are over your storage quota, Dropbox will stop syncing until you are below your limit.