"please select" or "you must select"? [duplicate]

This question already has an answer here:

It's a web-app. Something like GitHub for translators.

The user cannot proceed to the next action unless he selects some specified fields.

In general, which phrasing is better to use? We have a couple of similar sentences, for username, file size, etc. I want them all to be phrased in the same manner, but I struggle which one sounds better.

For example:

Please use at least 7 characters

You must use at least 7 characters

Passwords must be at least 7 characters long

Use at least 7 characters

or something different?