How to put a big equation in a single slide?

Much of the horizontal space is taken by the long square roots. Always presume that your audience is able to read, so you can use an abbreviation.

\documentclass{beamer}
\usepackage{mathtools}

\begin{document}

\begin{frame}

\begin{align}
& (\lambda_1-2n+l+4)(\lambda_1-n-l+2)\leq l(n-l) \notag
\\
\implies\quad
& \lambda_1^2+\lambda_1(6-3n)+(2n^2-2l-8n+8)\leq 0 \notag
\\
\implies\quad
& \Bigl \{ 2\lambda_1-\bigl(3n-6-\sqrt{D}\,\bigr)\Bigr\}
  \Bigl \{ 2\lambda_1-\bigl(3n-6+\sqrt{D}\,\bigr)\Bigr \} \leq 0 \notag
\\
\implies\quad
& \frac{3n-6-\sqrt{D}}{2}\leq \lambda_1 \leq \frac{3n-6+\sqrt{D}}{2} \label{whatever}
\end{align}
where $D=8l+n^2-4n+4$.

\end{frame}

\end{document}

A vertically centered equation number would be very ambiguous, so I chose to set it at the bottom. I also reduced the size of the parentheses; note the \, to separate the vinculum from the closing parenthesis.

In case of need, the middle long equation could be split using multlined (this is why I loaded mathtools.

\documentclass{beamer}
\usepackage{mathtools}

\begin{document}

\begin{frame}

\begin{align}
& (\lambda_1-2n+l+4)(\lambda_1-n-l+2)\leq l(n-l) \notag
\\
\implies\quad
& \lambda_1^2+\lambda_1(6-3n)+(2n^2-2l-8n+8)\leq 0 \notag
\\
\implies\quad
& \begin{multlined}[t]
  \Bigl \{ 2\lambda_1-\bigl(3n-6-\sqrt{D}\,\bigr)\Bigr\} \\
  \cdot \Bigl \{ 2\lambda_1-\bigl(3n-6+\sqrt{D}\,\bigr)\Bigr \} \leq 0 
  \end{multlined} \notag
\\
\implies\quad
& \frac{3n-6-\sqrt{D}}{2}\leq \lambda_1 \leq \frac{3n-6+\sqrt{D}}{2} \label{whatever}
\end{align}
where $D=8l+n^2-4n+4$.

\end{frame}

\end{document}

Finally, with the text typeset in sans serif, I'd avoid “l” as a variable.

Here's a solution suggestion that performs alignment on the inequality symbols.

Aside: The \bigg sizing instructions are, in my opinion, excessive; \big and \Big suffice completely.

\documentclass{beamer}
\begin{document}
\begin{frame}

\begin{equation}
\begin{aligned}
(\lambda_1-2n+l+4)(\lambda_1-n-l+2) &\leq l(n-l) \\
\implies \lambda_1^2+\lambda_1(6-3n)+(2n^2-2l-8n+8) &\leq 0\\
\implies \Bigl\{ 2\lambda_1-\bigl(3n-6-\sqrt{8l+n^2-4n+4}\,\bigr )\Bigr \} \quad&\\
\times\Bigl\{ 2\lambda_1-\bigl(3n-6+\sqrt{8l+n^2-4n+4}\,\bigr)\Bigr\} &\leq 0 \\
\implies  \frac{3n-6-\sqrt{8l+n^2-4n+4}}{2}&\leq \lambda_1\\
\text{and}\quad 
\frac{3n-6+\sqrt{8l+n^2-4n+4}}{2} &\geq \lambda_1\\
\end{aligned}
\end{equation}

\end{frame}
\end{document}

---

Addendum: Here's a second solution, inspired by @egreg's observation that a lot of space is taken up by the repeated term \sqrt{8l+n^2-4n+4}. Replacing it with the symbol D, rewriting the final row to use interval notation instead of a pair of inequalities, and aligning the rows on the \implies symbols instead of on the inequality symbols yields the following result:

\documentclass{beamer}
\begin{document}
\begin{frame}

\begin{equation}
\begin{aligned}[b]
&(\lambda_1-2n+l+4)(\lambda_1-n-l+2) \leq l(n-l)\\
\implies&\lambda_1^2-\lambda_1(3n-6)+(2n^2-2l-8n+8)\leq 0\\
\implies&\bigl( 2\lambda_1-(3n-6-D) \bigr) 
         \bigl( 2\lambda_1-(3n-6+D) \bigr) \leq 0 \\
\implies&\lambda_1\in\bigl[
         (\tfrac{3}{2}n-3)-\tfrac{1}{2}D, 
         (\tfrac{3}{2}n-3)+\tfrac{1}{2}D \bigr]
\end{aligned}
\end{equation}
where $D=\sqrt{(n-2)^2+8l}$\,.

\end{frame}
\end{document}

Inside a split you can specify an alignment point for each row using &, this greatly enhances the legibility of multi-line equations. Also, you'd have to split the very long lines into two lines as well. In the following I did so and move those continuing lines further to the right using \qquad.

\documentclass[]{beamer}

\usepackage[]{amsmath}

\begin{document}
\begin{frame}
\begin{equation}
  \begin{split}
    &(\lambda_1-2n+l+4)(\lambda_1-n-l+2)\leq l(n-l)
      \\
    \implies &\lambda_1^2+\lambda_1(6-3n)+(2n^2-2l-8n+8)\leq 0
      \\
    \implies &\bigg \{ 2\lambda_1-\bigg(3n-6-\sqrt{8l+n^2-4n+4}\bigg )\bigg \}
      \\
      &\qquad\cdot\bigg \{ 2\lambda_1-\bigg(3n-6+\sqrt{8l+n^2-4n+4}\bigg)\bigg \} \leq 0
      \\
    \implies  &\frac{3n-6-\sqrt{8l+n^2-4n+4}}{2}
      \\
    &\qquad\leq \lambda_1 \leq \frac{3n-6+\sqrt{8l+n^2-4n+4}}{2}
  \end{split}
\end{equation}

\end{frame}
\end{document}