remember THREE (3) important astronomical lengths:
UNIT
astronomical unit
light year
parsec
ABBREV.
AU
ly
pc
DEFINITION
avg. Earth - Sun dist.
dist. light travels in 1 yr
1 AU subtends 1'' at 1 pc
DISTANCE
156 x 106 km
1013 km
3.1 ly
MISCELLANEOUS
interplanetary length scale
(3 x 105 km/s)x(3.15 x 107s)
interstellar length scale

Other distances used are variants of these three:

intragalactic scale is kpc (kilo- = 1000)

intergalactic scale is Mpc (mega- = 106)

FYI, everything you always wanted to know about metric prefixes: