I suggest you load the tocloft package (\usepackage{tocloft}) and issue the instruction
\renewcommand{\cfttoctitlefont}{\Large\bfseries} in the preamble. To change the font size choose, e.g., \huge or \large instead of \Large.
Here's the full code that aligns the Table of Contents header to the left. (Note that while the header if flush-left, the entries are indented by a certain amount -- which is the default with or without the tocloft package.)
\documentclass[envcountsame]{llncs} \usepackage{llncsdoc,graphicx,acronym,url,verbatim} \usepackage{tocloft} \renewcommand{\cfttoctitlefont}{\Large\bfseries} \usepackage[hidelinks]{hyperref} \hypersetup{colorlinks,allcolors=black} \setcounter{tocdepth}{3} \setcounter{secnumdepth}{3} %\makeatletter %\renewcommand*\l@author[2]{} %\renewcommand*\l@title[2]{} %\makeatletter %\title{...} %\author{...} \begin{document} %\begin{center} %\email{...} %\end{center} \begin{abstract} Abstract here \end{abstract} \addtocontents{toc}{\vskip -1cm} \begingroup \let\clearpage\relax \tableofcontents \endgroup \bigskip\hrule % just to illustrate width of text block \section{A} \section{B} \end{document}