[Tex/LaTex] centering tikzpicture

Question

I have some problems concerning the command \centering. I want to put a tikzpicture in the center of the page but I can't do it. I tried with \centering and with \begin{center}...\end{center} but nothing happened.
I have also a problem with axis: I want to resize them but I don't know how to do it. Thank you in advance.
The code is:

\documentclass{article}  
\usepackage{pgfplots}  
\begin{document}  
\begin{center}  
\begin{tikzpicture}  
\begin{axis}  
[scale=2.5,axis lines=middle,  
enlargelimits,  
xtick={0,1.57,3.14,6.28},ytick={0.5,1},  
xticklabels={0,$\displaystyle \frac{\pi}{2}$,$\pi$, $2\pi$},  
yticklabels={$1/2$, $1$},  
xlabel=$x$,ylabel=$y$,axis equal]  
\addplot  
[domain=-2*pi:2*pi,samples=100,thick,blue]  
{(sin(deg(x)))^2+cos(deg(x))};  
\end{axis}  
\end{tikzpicture}  
\end{center}  
\end{document}  

Answer 1

Using showframe shows the issue -- the margins are not same on the left and right side. Using geometry this can be repaired, the given values are just guesses.

I did not address the axis scaling 'sub'question

\documentclass{article}  
\usepackage[lmargin=1.5cm,rmargin=1.5cm]{geometry}
\usepackage{pgfplots}  
\pgfplotsset{compat=1.13}
\usepackage{showframe} % just for showing the margins
\begin{document}  
\begin{center}  
\begin{tikzpicture}  
\begin{axis}  
[scale=2.5,axis lines=middle,  
enlargelimits,  
xtick={0,1.57,3.14,6.28},ytick={0.5,1},  
xticklabels={0,$\displaystyle \frac{\pi}{2}$,$\pi$, $2\pi$},  
yticklabels={$1/2$, $1$},  
xlabel=$x$,ylabel=$y$,axis equal]  
\addplot  
[domain=-2*pi:2*pi,samples=100,thick,blue]  
{(sin(deg(x)))^2+cos(deg(x))};  
\end{axis}  
\end{tikzpicture}  
\end{center}  
\end{document}