(1)

How to get rid of the comments shown by mathematica afetr each line which explain whatever command is typed ?

I'm not familiar with these. Can you provide a screenshot that highlights what you're talking about. Do you mean the "Predictive Interface" bar?

https://reference.wolfram.com/language/guide/WolframPredictiveInterface.html

If so, try searching online for "Mathematica Removing Predictive Interface". There are a couple of articles on this.

(2)

How to change the size of the characters of the menu ?

I would first try to change this with the options for your operating system - not for Mathematica. If the menu is too small, it's likely too small in other programs as well. Otherwise, you might be able to do this in the preferences dialog available under (Edit > Preferences) or on Mac under (Mathematica > Preferences).