Theoretically, there is no limit on the size of file you can upload to your LightRocket Media Manager platform.
As a rule of thumb we would recommend your hi-res files be not less than 3000 pixels on the longest edge. Files of this size will be perfectly usable for publishing in various formats.
We would also recommend avoiding uploading very large files using the drag and drop uploader. This is because the drag and drop uploader is dependent on the browser and a single interruption in the connection will cause the entire upload to fail.
For larger uploads - especially in the 100s of MBs - we would recommending using FTP as it is generally a more stable upload route.