Vias Size for Microstrip
You can design your via to minimize the discontinuity it introduces in your transmission line.

Basically this means balancing the inductance introduced by the via with the capacitance between the via and other nearby conductors (such as plane layer copper, etc). 

The ideal geometry won't just specify the via diameter, but also the keep-out diameter around the via, the diameter of pads on layers without connections (if used), and the location and diameter of vias for the return currents.

In addition to the required Z0, the ideal geometry will depend on the substrate Dk and which layers are being connected by the via (and thus how much stub is present on either side of the connection).

Designing the optimum geometry is generally a job for a 3-D EM simulation, although some tools (like the Saturn PCB tool) will give rough estimates. 

For 900 MHz, it's unlikely that optimizing the via geometry will be needed unless you are looking for extremely low reflection coefficients in your design.