I always wondered: Are there any hard facts which would indicate that either shorter or longer identifiers are better?

Example:

clrscr()

opposed to

ClearScreen()

Short identifiers should be faster to read because there are fewer characters but longer identifiers often better resemble natural language and therefore also should be faster to read.

Are there other aspects which suggest either a short or a verbose style?

EDIT: Just to clarify: I didn't ask: "What would you do in this case?". I asked for reasons to prefer one over the other, i.e. this is not a poll question.

Please, if you can, add some reason on why one would prefer one style over the other.