When we post a thread or response, what determines the amount of disk space is uses? is it basely strictly on the lenghth of the post or is it stored in file blocks with a fixed amount of space?
The amount of allocated disk space doesn’t directly correlate to the database size. This board uses a MySQL database that maintains a few big files that grow if needed, and space of deleted records just remains allocated until it gets reused or cleaned up manually, I think.
When you post, apart from creating some base data like indexing who posted which post when in what thread, your post text ends up in the big post database. All in all and on average, space allocation isn’t clustered in blocks, so mke it shrt, evry singl charctr counts. 
I was hoping this would be a way to prevent further exploding hamster incidents. By cutting back on excessive blank lines and multitudes of smilies, we could keep the servers from vapor-locking.
The space in the database used is not allocated in direct blocks, in general. However, a particular post or thread does add a row or rows to several tables. So there is both a “fixed” size per post (the mere fact that the post exists) and a “variable” size (the content of the post itself).
Thus, eliminating extra linefeeds, smilies, whitespace, etc. does reduce the database size. But not by very much, unless they are very excessive.