Leading on from the two dimensional world let us imagine that their world started as a point and expanded like a balloon being blown up. Their world would now be a two dimensional surface of a three dimensional sphere.
They are still two dimensional and so is their world - it is just curved.
Now if their astronomers wanted to explain the beginning of their universe they might come up with a kind of inflation or big bang theory and could see that everything is moving away from them by observation. To check this just put two dots on a flat balloon and blow it up. The dots move apart.
So they could easily come up with an expanding universe but could not define where it all started without adding a third dimension. The radius of their universe would be measured along this third dimension. They might theorise where and how their universe began, but would really have no idea.
They might guess that their universe is a particular size by observing the most distant object that they can see with their instruments. What they would really be seeing would be their real horizon. Their speculation would be strongly mathematically based BUT would, unfortunately be misguided as it would be based on incorrect assumptions.
Now let us look at ourselves. We know that time is a dimension (our fourth) but we cannot manipulate it. So let's imagine that the balloon now exists in 4 dimensional space and not three dimensional space as before. This new "hyper-balloon" would now have a three dimensional "hyper-skin" in which three dimensional creatures could exist. Kind of like our universe.
The astronomers in this "3-D skin" could look out into their universe with powerful telescopes and see what they believe to be the edge of the universe. They could theorise about its size and age BUT they could not tell exactly where the "Big Bang" occurred because it occurred in time and this is the dimension that we are destined to travel along in a predetermined direction and cannot manipulate.
They may use fancy mathematics and their observations to get a result of some sort but it cannot be correct because it is based on incomplete data. They can only see out to their 3-D horizon (if the instruments were good enough they might even see all the way around to themselves!). And, although it would be possible to measure the size of the universe using this data, there is as yet no one who has measured the true curviture of space in time and, unlike the early mathematicians measuring the earth, they do no have a "sun" to give them a clue.
If I ask where is the centre of the universe you can get no answer in the three dimensional sense. Point zero, the point of origin of our universe is only able to be described using a forth dimension and since we cannot as yet manipulate objects in four dimensions we cannot find the point at which the big bang occurred.
My guess is that the radius of our universe is along this fourth dimensional axis which could well be time. If the universe is expanding then we may be moving through time in a positive direction and as we have no way of getting outside the system to observe all four dimensions at once we are as yet unable to manipulate that fourth dimension.
From observation, it would appear that any creature can only manipulate and fully observe n-1 dimensions where n is the number of dimensions in which it exists. So, since we exist in 4 dimensional space, we can only properly move around in three of them. The fourth is, by its nature, read only.