It wasn't, according to our best current model. In that model the universe is spatially infinite and always has been, so it contains an infinite quantity of matter.
> wouldn't a limit on the universe size based on how much matter exists
There are mathematical models in which the universe has a finite size and contains a finite amount of matter. (These models are not completely ruled out by our current data, but they are considered very unlikely as compared to the ones in which the universe is spatially infinite.)
However, even in those models, the universe has no boundary: it is spatially a 3-sphere, which has a finite volume and no boundary similar to the way the surface of the Earth, a 2-sphere (at least approximately) has a finite area and no boundary.
It wasn't, according to our best current model. In that model the universe is spatially infinite and always has been, so it contains an infinite quantity of matter.
> wouldn't a limit on the universe size based on how much matter exists
There are mathematical models in which the universe has a finite size and contains a finite amount of matter. (These models are not completely ruled out by our current data, but they are considered very unlikely as compared to the ones in which the universe is spatially infinite.)
However, even in those models, the universe has no boundary: it is spatially a 3-sphere, which has a finite volume and no boundary similar to the way the surface of the Earth, a 2-sphere (at least approximately) has a finite area and no boundary.