There is no set rule to how big you should be. People generally discourage folks from using AAS "too early" for a couple main reasons:
If you've packed on a good amount of mass "naturally", that shows that you know how to train and eat properly. This means you are MUCH less likely to lose most if not all your gains shortly after the cycle.
Also, when you first start lifting, gains typically come much easier than they will later. If you're still in this "honeymoon period", it isn't usually worth it to be dealing with all the risks and side effects that come along with AAS use.
Other than that, people just like to tell others that they're "too small" because it makes them feel macho and AAS users on the internet are typically elitist a**holes who think only "certain" people are "qualified" to use AAS.
As for your PCT, you'd just use a "standard" protocol. A dianabol solo cycle isn't going to require anything special. If you don't know what I mean when I talk about a "standard" PCT, then you really haven't even scratched the surface on the research you need to do first. I'm guessing this might be the case given the vagueness of your question - when people have done research they typically will post the cycle that they came up with themselves based on their research and are looking for helpful critiques.
