documentclass[12pt]{book}

usepackage{polyglossia}

setdefaultlanguage[numerals=maghrib]{arabic}
setotherlanguage{english}

setmainfont{Traditional Arabic}
newfontfamilyenglishfont{Times New Roman}

begin{document}

pagestyle{empty}

begin{english}
noindent
One can write multi-lingual documents, and math equations, by using the polyglossia package, and compile it with XeLaTeX, in several sizes. The basic size is 12pt.
end{english}
hrule

{centering

LR{Text in 14pt is obtained by using the command $backslash$large:

{large A latin text, in size 14 pt.}

An arabic text, in size 14 pt, is the following centered text:}

 {large
نص عربي في حجم 14.
}

hrule

LR{Text in 17pt is obtained by using the command $backslash$Large:

{Large A latin text, in size 17 pt.}

An arabic text, in size 17 pt, is the following centered text:}

 {Large
نص عربي في حجم 17.
}

hrule

LR{For larger sizes, we use the command, for example, $backslash$fontsize{18}{22}, which puts the size to 18pt for arabic and english characters, but has no effect under the package polyglossia, else if we put the characters under the commands \
$backslash$begin{Arabic} ...$backslash$end{Arabic}, and $backslash$begin{english} ...$backslash$end{english}, respectively. I give examples of compilations:

hrule
{fontsize{18}{22} A latin text, under the command $backslash$fontsize{18}{22}, and outside $backslash$begin{english} ...$backslash$end{english}.}
hrule

 {fontsize{18}{22} begin{english}
 A latin text, under the command $backslash$fontsize{18}{22}, and $backslash$begin{english} ...$backslash$end{english}.
 end{english}}
hrule

 Arabic text, under the command $backslash$fontsize{18}{22}, and outside $backslash$begin{Arabic} ...$backslash$end{Arabic}:
}

{fontsize{18}{22}
نص عربي في حجم 18.
}

hrule
 LR{ Arabic text, under the command $backslash$fontsize{18}{22}, and $backslash$begin{Arabic} ...$backslash$end{Arabic}:}

{fontsize{18}{22}
begin{Arabic}
نص عربي في حجم 18.
end{Arabic}}

par}

end{document}