Asking what the self is is like asking what the weight is.

Usually when we ask about weight we don’t mean what you are at any moment since weight changes daily and even hourly. It usually means what it is on average. Similarly do we mean by self what you are at any given moment or more generally ?
Weight fluctuates within certain parameters depending on age(your mind is changing a lot until a certain age also), etc
Some things like bone density muscle mass will effect the weighy(just like some seemingly unchanging characteristics of the self seem static.,they may be genetically based just like some things in body weight are)

I can tell you my weight at this very moment. It will be effected by things like water retention, if and what I ate earlier etc. I can give you the exact parameters of my self at this moment but it will be effected by things like mood, level of physical energy etc. To get a more meaningful answer to how much I weight, I must be observed and measured over a longer period of time to get some sort of average. This is the same with the self.