tiny plot of land; narrow strip of land
Literally 'land as big as a cat's forehead'; a set phrase describing a very small piece of land, often used humorously or self-deprecatingly.
He built a small house on a tiny plot of land.
Even on this postage-stamp-sized piece of land, I can enjoy gardening.
狭い is a general adjective for 'narrow' or 'small' in area, while 猫額大の土地 is a specific, vivid set phrase for a very small plot of land.
The phrase literally means 'land as big as a cat's forehead', using the image of a cat's small forehead to humorously emphasize the tiny size of a plot of land.