What is the units of ArrowStyle's instance? points or pixels or other?

I want to know how can I exactly control the bracket’s width or length ? for example, I want to let length from 6 to 7 in my red bracket.
what is the units of bracket’s width or length?

My code as follows:

import matplotlib.pyplot as plt
fig, ax = plt.subplots(figsize=(8, 8), dpi=72)
ax.set_xlim(0, 8)
ax.set_ylim(0, 8)
ax.plot([3, 4], [2, 2], marker=“o”, linestyle=“None”)
ax.annotate(“”, xy=(3, 2), xytext=(4, 2),
arrowprops=dict(arrowstyle=“]-, widthA=2, lengthA=2”, connectionstyle=“arc”))
ax.annotate(“”, xy=(3, 5), xytext=(3, 6),
arrowprops=dict(arrowstyle=“]-, widthA=2, lengthA=2”, connectionstyle=“arc”, color=“r”))