Qt's API for manipulating files (e.g. QFile) by itself imposes no unreasonable limits beyond those of the underlying platform. For instance, all positions in QIODevice-derived classes are represented by qint64. In theory, the API can cope with files up to 2^63 bytes long.

However, you have to be careful with how you use this API if you want your application to scale. For instance, if you use a QNetworkReply to download a file and write it to disk with QFile, make sure you read and write in blocks whose size is bounded; avoid methods like QIODevice::readAll().