• Does it matter if the file will be downloaded vs. shown in browser?

A good thing to keep in mind is the performance and user experience of viewing the file in browser vs on computer. Depending on your audience people use different browsers and older browsers which are not good at handling a decent PDF form too. After debating on this, you should decide which route to follow. Remember performance will (almost always) be better locally, just see how much of a difference it is.

• Only show it for files that are exceptionally big (e.g. a 10 MB PDF, a 3 MB JPG)? Exceptionally small (e.g. a 500 KB MP3), too?

Consistency is a key element of UX. Since you have files on both ends, I would recommend showing the size all the time.

• Exact vs. rounded file size?

For the most part I do not see the problem with rounded file sizes, specially if your audience is non technical.

• Only use one unit for all downloads (e.g. Megabyte only: 1 MB, 0.1 MB, 0.01 MB, 1280 MB)?

While using a single unit has it merits, I would do some user testing to decide on this. I feel that just limiting to 2 units might also be a good alternative.

• Should the file size information be part of the linked text, or could it appear as unlinked text after/below the link?

As long as it is clearly visible before the user starts the download, it should be fine outside the anchor.

• Should the file size information get a label or is it clear by context (thanks to using parenthesis and a file size unit?)?

Being explicit never hurts, mention file size ... if you have the screen space / no other constraints preventing you from doing it.

All my answers are observations/opinions. Do user testing with your audience to finalize the direction.

• Does it matter if the file will be downloaded vs. shown in browser?

A good thing to keep in mind is the performance and user experience of viewing the file in browser vs on computer. Depending on your audience people use different browsers and older browsers which are not good at handling a decent PDF form too. After debating on this, you should decide which route to follow. Remember performance will (almost always) be better locally, just see how much of a difference it is. Nielsen article on similar topic.

• Only show it for files that are exceptionally big (e.g. a 10 MB PDF, a 3 MB JPG)? Exceptionally small (e.g. a 500 KB MP3), too?

Consistency is a key element of UX. Since you have files on both ends, I would recommend showing the size all the time.

• Exact vs. rounded file size?

For the most part I do not see the problem with rounded file sizes, specially if your audience is non technical.

• Only use one unit for all downloads (e.g. Megabyte only: 1 MB, 0.1 MB, 0.01 MB, 1280 MB)?

While using a single unit has it merits, I would do some user testing to decide on this. I feel that just limiting to 2 units might also be a good alternative.

• Should the file size information be part of the linked text, or could it appear as unlinked text after/below the link?

As long as it is clearly visible before the user starts the download, it should be fine outside the anchor.

• Should the file size information get a label or is it clear by context (thanks to using parenthesis and a file size unit?)?

Being explicit never hurts, mention file size ... if you have the screen space / no other constraints preventing you from doing it.

All my answers are observations/opinions. Do user testing with your audience to finalize the direction.