Governor limits include
Maximum size of callout request or response (HTTP request or Web services call) - 3 MB
Does this include downloading files via HTTP from external servers? Will those files also be restricted to 3 megabytes?
If you are using an
Furthermore, this callout counts towards the total Heap Size limit for a single Apex transaction, which is 6MB (or 12MB if you're in Batch Apex, which can make callouts as well if properly annotated). So, for instance, if you need to post 3 large documents, say 1.9 MB a piece that you're generating from within Apex to an external service, you could do 3 separate callouts within a single Apex transaction, and you would presumably be at only around 5.7 MB for your total heap size, and since no single callout generated a request body of greater than 3 MB, you should be fine.
If you have control over the remote endpoint that you are downloading / posting files to, you might want to take a look at leveraging the