Does the basicity decreases as going down the column and right across the periodic table? How does the size of an element or electronegativity apply to strength of basicity and acidicity?

Answer 1

Yes, the trend is as you have it.
The strength HF<HCl<HBr<HI so you see the acidity increases as size increases.
For electronegativity, acidity increases with electronegativity; i.e., NH3<H2O<HF