Not even 3 inches tall! 
There are no units - it could be 3 metres!!
You need to be more observant. There are 8 division marks meaning it is inches. If it was metric there would be ten divisions. 
And you should not just assuming its inches, just going on previous experience.
Even if, as a standard, thats how we divide up metric / imperial, from that photo you still do not know that its inches.
Without units, it is useless - as my Applied Engineering Sciences tutor always used to say (if, in class, we ever gave an answer without units - he would always respond - "400 what? Elephants per fortnight?").
I mean, technically, how do you not know its measuring yards...?

Yes, the classic and you're right I can't prove it, but what's the likelihood that someone made a tape measure that's a foot wide?
So we've either got the following options;
1) it's an unknown scale and we're looking at an image that's impossible to determine it's size, overlooking all other factors on the image.
2) someone went to great lenght, it was purposely designed to fool the viewer in an attempt to show faulty logic in some test of reasoning.
3) It's just a cheap old American dime store tape measure, where no one gave a rat's
ass about the metric system.
So you either make a pretty safe assumption that it's inches, or you never make an assumption about anything and therefore anything is possible, like that scale being 3 nanometers or 3 light years in lenght.
I'm happy with making certain assumptions based on observation.