D=left{ xinmathbb{N}|1leq xleq 100 right}

documentclass{article}

usepackage{amsmath}
usepackage{amssymb}

newcommand{numberset}[1]{mathbb{#1}}
newcommand{nat}{numberset{N}}}

begin{document}

[
D = {, xinnat mid 1le xle 100 ,}
]

end{document}

documentclass{article}

usepackage{amsmath}
usepackage{amssymb}
usepackage{mathtools}

newcommand{numberset}[1]{mathbb{#1}}
newcommand{nat}{numberset{N}}

DeclarePairedDelimiterX{Set}[2]{}{%
  , #1 ;delimsizevert; #2 ,
}

begin{document}

begin{gather*}
D=Set{xinnat}{1le xle 100}\
D=Set[big]{xinnat}{1le xle 100}\
D=Set[Big]{xinnat}{1le xle 100}\
D=Set[bigg]{xinnat}{1le xle 100}\
D=Set[Bigg]{xinnat}{1le xle 100}\
D=Set*{xinnat}{1le xle frac{200}{2}}
end{gather*}

end{document}