You can find the answers just by searching the forum in under five minutes, but to answer very briefly (without going into exceptions and detailed count of posts):

-   Activity is derived from the number of posts, capped at 14 max. in a two week period (see FAQ: Everything you need to know about forum 'activity, account ranks and merit) for details.
-   Activity is based on your posts over 2 week periods. No posts, then no activity increase.
-   A post that is a thread opener is just that, a post, and therefore counts towards activity just as any other post.
-   Length of the posts does not alter any counter at all, nor matter.

Of course crappy post may get deleted, and consequently the activity count will decrease.