I saw someone not set the length in a tutorial but it was specifically for counting the total number of users and just set to auto-increment. I've been of the habit of always specifying a length because I thought it was mandatory, but I wanted to ask if I can leave it blank unless it specifically a date or pin number etc where the length is always set. (I used to set it as 11 digits or more if I wasn't sure)
Every integer field defaults to 11 when left blank so you can leave it.
No, you don't have to specify a length for integers. I have never done that.
There are different integral data types with different sizes, so you can just choose the one with the capacity that you need.
来源：https://stackoverflow.com/questions/7053596/do-i-have-to-specify-integer-length-when-creating-an-id-field-in-mysql-through-p