Is a magiphage's size effected by the amount of physical mass that went into their form, the amount of magical energy, both, or neither? What/who are the largest and smallest magiphages in your canon?
The biggest thus far is Roosevelt, but they can be a tiny bit bigger, and the smallest is Wax, but they can be much smaller. My mental rule of thumb size range is between the sizes of a grizzly bear and a field mouse.
Their size is determined by the characteristics magic decides they are supposed to have - that could be due to the physical materials that went into their development or it could be determined by the ideas in the bits of magic that went into their creation. Either way, while the source of some of it might be physical stuff that was there, they are still traits written in a kind of magic dna that decides how their body works, how to build it to begin with, and how to maintain it on an ongoing basis from then on.
If they achieve life with all the material and energy needed to be as big as magic says they ought to be, they’ll be born fully formed, if not, they will grow, like a baby growing into an adult. So not having everything there isn’t necessarily an impediment, as long as there was enough in one place to spark the chain reaction