猫の額 (ねこのひたい, neko no hitai) literally means “a cat’s forehead”. Figuratively, it indicates (with a touch of humour/sarcasm) that the area of something is very narrow/small, like a cat’s forehead.
It is usually used as ‘猫の額ほどの’ meaning “as narrow as a cat’s forehead”. For some reason, it is dominantly used to describe a small garden.
I grow vegetables in my very small garden.
(とても)狭い, (very) narrow
ウサギ小屋(ごや)のような “like a rabbit hutch” (very small)