Below figure shows the plot of which arrow head is very small...

I tried below code, but it didnot work... it said " raise AttributeError('Unknown property %s' % k) AttributeError: Unknown property headwidth"...

xyfrom=[10,620]
xyto=[130,620]
ax.annotate("",xyfrom,xyto,arrowprops=dict(arrowstyle='<->',linewidth = 2,     headwidth=10,color = 'k'
))
ax.text((xyto[0]+xyfrom[0])/2-15,(xyto[1]+xyfrom[1])/2+10,"headwidth is too    small",fontsize=24)

I believe it's because you need to give your arrowstyle arguments inside a string. Try this:

 arrowprops=dict(arrowstyle='<->, head_width=10', facecolor='k')

, notice how this is a full string:

 '<->, head_width=10'

It's a really strange choice in matplotlib, one I really don't understand why should it be this way. In any case see if solves your problem.