I assume you start at two letters, as in your example. I'm using the circa 2013 Words With Friends dictionary.
If you can only add to the right end of the word, 8-letter words are possible:
ba
bar
barb
barbe
barbel
barbell
barbells
ma
max
maxi
maxim
maxima
maximal
maximals
pa
pas
past
paste
paster
pastern
pasterns
re
rep
repo
repos
repose
reposer
reposers
If you can add to either end, 9-letter words are possible:
id
aid
aide
aider
aiders
raiders
braiders
abraiders
la
lap
laps
lapse
elapse
relapse
relapser
relapsers
in
pin
ping
aping
raping
craping
scraping
scrapings
at
eat
eath
heath
sheath
sheathe
sheather
sheathers
is
ais
rais
raise
raiser
raisers
praisers
upraisers
If you can add at any point, 11-letter words are possible:
pi
pig
ping
oping
coping
comping
compting
competing
completing
complecting
