As far as moving to the larger sizes goes, pretty much let (dis)-comfort be your guide.
Early on, like the stage where you are, Deb, depth is the most important thing to concentrate on. It is the easiest thing to lose and the hardest to regain. So starting with your most comfortable dilator and spending a good period of time making sure you stretch your vagina to it's maximum depth is best.
Then start trying to use the larger sizes as you can endure the discomfort. If using the larger size hurts too much or if you start bleeding excessively, then stop.
In my experience, after a couple of weeks, I would start with my smallest dilator for about 10 minutes and then insert the next larger size for about 5 minutes. Or at least try to. At first I could only endure the pain for a couple of minutes. But after a few days, I could keep the larger one in for longer and longer periods of time.
After the first few weeks of this, I became stretched enough that I could start with the larger dilator and get my maximum depth at the same time and I would spend my entire time with the larger dilator.
I would then try to move up to the next larger size using the same method.
These days for me, I dilate every week or so, and I just use my smallest one. It's painless, and sometimes fun. But I find that I have to use the same method to re-stretch myself to regain my width. And while it does pinch a bit working "Big John" and his larger brothers in there I can do it with little discomfort.
But I would add one piece of advice that Doctor McGuinn left me with, and it does appear to be true. Most men will be no wider than your second largest dilator. And you can have very enjoyable intercourse if you are just using the first two dilators, and "organic" dilation. It is, generally, not necessary to always work your way up to being like the Alaska pipeline, unless you need to.
If you put it in the perspective of what happens with a cis-woman in a relationship, her body will stretch to best accommodate her partner. And that is what you are doing as well.
Depth and comfort are your best guides going forward.
-Sandy