Metal and wood caskets typically have similar exterior dimensions, although you can expect wood caskets to be slightly smaller.
However, the fact that metal can be thinner while maintaining its structural integrity means that metal caskets typically have a slightly roomier interior. Conversely, because the panels used to construct wood coffins must be thicker, their interior is smaller.
Metal caskets are often a good option to explore in the case of slightly larger or taller bodies; the interior bedding can be adjusted to create even more space.