It's just under 20cm (8in) long and very, very sharp.
My grandmother used to take old cotton clothes that were too ragged or torn to wear, and cut them up into strips maybe 1.5cm (½in) wide and 20cm (3in) long. She'd tie a knot at one end of each strip, sort them out by colour, then when she had enough she'd take a piece of sacking and use this tool to pull the strips of cloth through the sacking. She'd arrange it to make patterns, mostly abstract but sometimes representational (flowers and easy images, rather than animals or anything complicated).
I often saw her doing this when we were staying at my grandparents, while we were watching TV or she was listening to the wireless. It was quite a long and painstaking task, but the end results looked really rather good — I was certainly impressed as a child that she'd made some of the rugs in the house herself.
I've no idea if this is what the tool was supposed to be used for, but that's what it was used for.
I don't think I'll be trying it myself (and not just because, unlike my grandmother, I can afford to buy rugs ready-made).