I don't really understand your talk about fertility but I think you just want the large central areas not to be deserts?
Rain originally comes from the sea and the temperature of the sea has a large impact. Place the center of the continent on the equator so that it relies on rains from warm tropical ocean. You can boost this by creating a large Gulf on the western coast.
Hadley cells are also a thing. Placing the center on the equator is the main thing but you will want to keep on eye on what is where dry air descends as well. Simplest solution would probably just put an inland sea such as the Mediterranean in this "wasted" space. Just because you have a large continent does not mean all of it has to be above sea level. Alternately you can have water from North or South flow here. Or fill it with mountains.
Rain shadows from mountains also matter. You do not want large coastal mountains that could stop the rains from going inland. Realistically, if your continents have been stuck together more less permanently, which seems to be your idea, the mountains will have eroded. Maybe you can have the coastal mountains off-shore as islands creating nice inland seas? Or you could have an inland sea inside the coastal mountains connected to the ocean by straits.
Where water goes is also important. Central Asia took a long time to dry out because water flowed to large salt lakes and the trapped moisture amplified monsoons bringing in seasonal rains. Less water is removed from the center by rivers, less water rains need to bring in to keep it "fertile". So maybe have large areas of the continent drain to large salt lakes?
A Pangaea that does not have a central desert would probably have powerful monsoons. And that is a plural. A monsoon is a seasonal pattern so it probably is not that strong in the equatorial center but it would be a big help keeping Northern and Southern areas wet, if they are large enough.
The bottom line is that fertility depends on the climate and that depends on what geography you give your continents. Not just on a single variable such as continent size.