[MrGLaDOS]

Actually not necessarily. Many light, cheap and small antennas can also work.

See p.8 and 10 for those used in the current state of the art telescope at 160MHz https://www.aanda.org/articles/aa/pdf/2013/08/aa20873-12.pdf And https://cdr.skatelescope.org/#photos?lfaa for those used in the SKA, one of the largest future radio telescopes that is still under construction.

Getting the distances between the telescopes is actually easier in space. Both distance measuring and datatransmission could be done with lasers when there is a clear line of sight. Moreover, once an orbit is established, the laws of Kepler are 'followed' and predicting their mutual distances is something we can do extremely well. On Earth with very long baselines it is much trickier and things that need to be taken into consideration are cablelength differences due to temperature changes, tides and continental drift. (Continental drift is actually measured with radio telescopes: in the reverse problem when the location of a set of sources on the sky is known to high precision one can establish at what speed the distances between the telescopes is changing.)