Some brands will fit better than others. For example, Bellissimos and Comficares can collect 1.3 liters while sitting. Diapers cut ont he same machine seem to have similar results.
I've had other brands just immediately leak on first wetting while sitting, and I simply don't use them any more. Fit is important.
As far as sitting not being safe? I think that depends on whether you are willing to augment your diaper. I use Bambino Quadros in the front of my diaper during work. With one, I can get close to 1.7 liters. With 2, over 2 liters. The only real downside here is this adds a huge amount of bulk to the point it looks like there is a balloon in the front of your pants, but it does work.
Here is a recommentation. Try a smaller fit. Try to get a brand with the absolute best (tallest, not leaky) leak gaurds, and then add some kind of booster up the front. Some brands, like the Snuggies, simply just don't have the tall leak gaurds needed for a booster and are not good for augmentation.
Maybe someone can chime in on which brands have fantastic leak gaurds. I certainly can't remember from the 10 brands I've tried, other than Bellissimo and Comficare.