Yeah, check with your own surgeon. Brassard was very specific about when to increase dilator size, although he does also say it's OK to go back down to more frequent sessions with a smaller dilator if the big one is too painful/difficult. (Then try again in a week or two.)
I've included his instructions below because they're very specific; we aren't told to leave it up to ourselves and when we're "ready" until after the one-year mark and reducing to once a week. I do not suggest following his instructions without consulting your own surgeon, mind you; this is just for an example of what a precisely laid out size increase schedule looks like.
(#1 is the smallest dilator, #2 the next size up, and so on. I do hear that these days he's dropped the #1 size altogether and starts with #2 and #3 now.)
"At home, you should dilate 4 times per day with, #1, #2 for 5, 15
minutes for one month.
Dilate 3 times per day with #1, #2, #3 for 5, 10, 15 minutes for two months.
Dilate twice a day with #2, #3, #4, for 5, 10, 15 minutes for 3 months
Dilate once a day with #3, #4, for 5, 15 minutes for 6 months.
Slowly increase the interval of time between each dilation until you
reach one dilation per week with #4, for 15 minutes for the rest of
your life.
If you have difficulties with a bigger size dilator, wait before you
are comfortable with this one before using a bigger size. You can go
back to a smaller size and increase the frequency for a few days."