My choice for bigger and more frequent poos is a combination of Metamucil and Miralax.
I'd start with the Metamucil alone. I've found that the fiber thins are a pretty pleasant way to take this stuff. The Metamucil powder is disgusting to take because it starts to swell up as soon as it's mixed with water. It's like trying to drink a glass of K-Y. The thins are crisp little cookies. If you don't read the package you probably wouldn't know they are a high fiber supplement. Start with eating about 3 packets a day for a couple of days. If you want bigger poos, increase the daily dosage. For me, eating 3 packets a day in addition to what I normally eat, results in poos of about a double hand full daily.
If you need something to give you an urge to poo in you diaper, a dose of Senna at bedtime will usually get things moving in the morning.
The Metamucil will usually soften your poo somewhat, and, personally, I am not a fan of really soft poo passed into a diaper. You can add a daily dose or two of Miralax, a completely tasteless poo softener. This stuff really works... if you take a bit, it'll soften a bit. If you take a lot, you'll poo soup.
With this game, there is no 'one size fits all'. Pick a starting point and adjust until you get the results you like. Also, it takes several days for a change in what you take to wind up in you diaper.