\documentclass[a4paper,11pt]{book}
\usepackage[polish]{babel}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{geometry}

\geometry{width=126mm}


\newtheoremstyle{scstyle}{3pt}{3pt}{\itshape}{0pt}{\scshape}{.}{ }{}
\theoremstyle{scstyle}
\newtheorem{wniosek}{Wniosek}[chapter]

\begin{document}

\chapter{Test środowisk}
\label{cha:test}

\section{Test środowisk}
\label{sec:test}

\begin{wniosek}
\label{wn:dlugosc:wektora}
Jeśli $\mathbf{u} = \alpha\mathbf{v}$, to $|\mathbf{u}| = 
|\alpha||\mathbf{v}|$.
\end{wniosek}

\end{document}