Our everyday notions of space and time cease to be valid. Although we have little knowledge of the Universe before the Planck time, we can calculate when this era ends and when our physics begins. This occurs when the Universe is at the Planck scale in its expansion. One could, in theory, measure the size of the Universe much like you measure the radius of the Earth. The Universe expands from the moment of the Big Bang, but until the Universe reaches the size of the Planck scale, there is no time or space.
Time remains undefined, space is compactified.
During the Planck era, the Universe can be best described as a quantum foam of 10 dimensions containing Planck length sized black holes continuously being created and annihilated with no cause or effect. In other words, try not to think about this era. One of the reasons our physics is incomplete during the Planck era is a lack of understand of the unification of the forces of Nature during this time. At high energies and temperatures, the forces of Nature become symmetric. This means the forces resemble each other and become similar in strength, they unify.