Some people prepare a letter of recommendation and ask the advisor to send that to the destination university. This is by no means advisable because, if you are not able to write a professional text, the decision makers of the destination university cannot properly judge your abilities and you may lose your chance of admission.
If the professor or the advisor asked you to write a letter of recommendation due to shortage of time, ask him to read the text at the end, at least send your letter of recommendation to an editor to revise it. Lengthy texts are not recommended because they are too boring and important notes cannot be distinguished easily. Also, a short text may induce the feeling that the author does not have the incentive to write it. Therefore, almost one page will be enough for this purpose. Some professors write the letter of recommendation themselves and send it directly to the destination university. In this case, check the letter out make sure all key issues are mentioned fully.