# What is the length in millimeters of a crystal of copper sulfate that is 0.250 inch long?

$1$ $\text{inch}$ $=$ $25.4$ $m m$.
So, $0.25$ $\text{inches}$ $\times$ $25.4$ $m m \cdot {\text{inch}}^{-} 1$ $\approx$ $6$ $m m$. This is a big, macroscopic crystal.
I am reminded of Cockney rhyming slang where $\text{half-inch}$ means $\text{pinch}$, to rob someone. "Here, someone's half-inched my wallet!"