$\DeclareMathOperator\diam{diam}$Looking for a proof in the literature of the following lemma:
Let $K\subset\mathbb{R}^d$ be a bounded domain. Let $P_X$ be a finite dimensional subspace of $\mathcal{H}^k(K)$ then
$$\left\|v\right\|_{\mathcal{H}^k(K)} \leq C \diam(K)^{-k} \left\|v\right\|_{L_2(K)}$$
for all $v\in P_X$ and where the constant $C$ does not depend on $\diam(K)$.
There is a proof in The mathematical theory of finite element method by Susanne C. Brenner, L. Ridgway Scott (p111), but they do not check if the constant $C$ is independent of $\diam(K)$.