These big ones take a long time to mature so the smaller divisions will be setback 5 years or more. I have never divided my hosta and they get bigger and more beautiful every succeeding year. Instead, if I have another area in my garden to plant hosta I purchase another variety to plant since new or one from a cutting, the time to maturity will be the same. To answer your question specifically though, from what I know, the best time is in the spring when the leaf spikes are just popping through. Dig out or in the case of your large ones, dig around the whole plant, wash the soil off and divide the plant 3 or 4 unfrilled leaf spikes to a crown and replant. Once in the ground water very well as one would do with bog plants. You can also divide in the fall but the weather changes very quickly with regards to its down swing and there is greater risk of not being successful.

thanks for reply, i really do hate to divide them they are so beautiful but also so big, maybe I'll just let them be? have till spring to decide!

Seems you have what gardeners strive for and from what you say they are not torn to shreds by slugs.

surprising but i never have had problems with slugs , now ants i do have problems and of course my 3 rabbits but they do not like hostas yet?