Not sure about the site but in general publishing the usual format is:

- double line spaced
- 'courier style' font (i.e. courier new) in 12pt

The reason for this is not so much for the look of the typescript, however, but more for the convenience of the editor because the above format both allows notes in between the lines and also easy word counting (any courier font should have even letter sizes, unlike fonts like TNR where some letters i.e. i, l, j have less space. This means if you use 12pt and double line spacing you should have very close to 250 words per page which is the same as what you normally get on a published novel so an editor can quickly guess the word count by counting the number of pages).

However, for online publication, you need something easy to read so whatever works for you, I think... I don't think the library mods change what formatting the writer uses (don't seem to, anyway... as there are many different formats)