Tails is a live system. So if you are not writing your download on an external storage, the downloaded content is stored in RAM. And you are thus limited by the amount of RAM the machine has.

I am unsure what is a "download manager" in your mind, but almost all software in Debian can be installed in Tails, so you are free to pick the one you prefer the most.

Please do not put load on the Tor network without thinking about whose life depends on it… or at least plan to contribute by running a high bandwidth node.