  High cadence transient surveys are able to capture supernovae closer to their
first light than before. Applying analytical models to such early emission, we
can constrain the progenitor stars properties. In this paper, we present
observations of SN2018fif (ZTF18abokyfk). The supernova was discovered close to
first light and monitored by the Zwicky Transient Facility (ZTF) and the Neil
Gehrels Swift Observatory. Early spectroscopic observations suggest that the
progenitor of SN2018fif was surrounded by relatively small amounts of
circumstellar material (CSM) compared to all previous cases. This
particularity, coupled with the high cadence multiple-band coverage, makes it a
good candidate to investigate using shock-cooling models. We employ the
SOPRANOS code, an implementation of the model by Sapir & Waxman and its
extension to early times by Morag, Sapir & Waxman. Compared with previous
implementations, SOPRANOS has the advantage of including a careful account of
the limited temporal validity domain of the shock-cooling model as well as
allowing usage of the entirety of the early UV data. We find that the
progenitor of SN2018fif was a large red supergiant, with a radius of
R=744.0_{-128.0}^{+183.0} solar radii and an ejected mass of
Mej=9.3_{-5.8}^{+0.4} solar masses. Our model also gives information on the
explosion epoch, the progenitor inner structure, the shock velocity and the
extinction. The distribution of radii is double-peaked, with lower radii
corresponding to lower values of the extinction, earlier recombination times
and better match to the early UV data. If these correlations persist in future
objects, denser spectroscopic monitoring constraining the time of
recombination, as well as accurate UV observations (e.g. with ULTRASAT), will
help break the radius-extinction degeneracy and independently determine both.
