Measuring the size and the geometry of Universe
The relative recent WMAP result shows, the Universe seems infinite.

However, the "is the Universe infinite" question has a hidden problem: it doesn't matter, how far away can we see, we can't be sure, if anything out of it is finite or not. :-) Thus, it is about a definitely unknowable thing.

The WMAP measured essentially the curvature of the Universe and it found a zero curvature (i.e. it seems to be planar). If there is a minimal curvature, below a measurement precision, and we have a cyclic, spherical geometry, then its radius must be at least 300billion light years.