Some more pairs of opposites, each with an identical number of letters:

force-allow
frozen-thawed
fixed-broke
prolong-shorten
lit-dim
temporary-permanent
clean-dirty
live-dead
mom-dad
rare-done
raise-lower
skinny-rotund
punish-reward
order-chaos
faith-proof
listen-ignore
above-below
rough-slick
cut-fat
home-away
gross-small
grand-plain
more-less
mean-nice
city-farm
her-him
daring-demure
menace-savior
failure-success
routine-unusual
sink-rise