[Tex/LaTex] backslashbox is too wide

Question

I feel the first column in my longtable is too wide. Please see the figure below

My code is:

\begin{center}  
\begin{longtable}{|c|c|c|c|c|c|}  
\caption{Error rates for SVM and 100 random counts. \label{tab:2}}\\  
\hline  
\backslashbox{Concepts}{Train sizes} & 10 & 100 & 1000 & 10000 & 100000\\  
\hline  
1 & 0.49 (0.00) & 0.49 (0.00) & 0.47 (0.00) & 0.43 (0.00) & 0.33 (0.00)\\  
\hline  
\end{longtable}  
\end{center}  

I wonder if there is some way to reduce the width of the first column?

Thanks and regards!

---

EDIT:

As Hugo suggested, I tried to specify the width for the first column.

\begin{longtable}{|p{50pt}|c|c|c|c|c|}  

But it appears messed up

Answer 1

Try

\backslashbox{Concepts\kern-1em}{\kern-1em Train sizes}

or

\backslashbox{Concepts\kern-2em}{\kern-2em Train sizes}

or

\backslashbox{Concepts\kern-3em}{\kern-3em Train sizes}