The 'Observable Universe' is limited by the speed of light and the length of time that it has been in existence. It's possible that the 'Observable Universe' is the real size, or, because light has a finite speed, there may be more universe 'outside' the limit we have so far been able to observe.
If the observable universe encompasses the entire universe, we may be able to determine the structure of the entire universe by observation. However, if the observable universe is smaller than the entire universe, our observations will be limited to only a part of the whole, and we may not be able to determine its global geometry through measurement. From experiments, it is possible to construct different mathematical models of the global geometry of the entire universe, all of which are consistent with current observational data; thus it is currently unknown whether the observable universe is identical to the global universe, or is instead many orders of magnitude smaller.
The 'shape' of the universe is also unknown. Recent measurements indicate that it's probably 'flat' (i.e. with zero curvature).
The exact shape is still a matter of debate in physical cosmology, but experimental data from various independent sources (WMAP, BOOMERanG, and Planck for example) confirm that the universe is flat with only a 0.4% margin of error. Theorists have been trying to construct a formal mathematical model of the shape of the universe.
Also see: Expansion of the universe