  Epitaxial growth of WTe$_2$ offers significant advantages, including the
production of high-qualityfilms, possible long range in-plane ordering and
precise control over layer thicknesses. However,the mean island size of WTe$_2$
grown by molecular beam epitaxy (MBE) in litterature is only a fewtens of
nanometers, which is not suitable for an implementation of devices at large
lateral scales.Here we report the growth of Td-WTe$_2$ ultrathin films by MBE
on monolayer (ML) graphenereaching a mean flake size of $\cong$110nm, which is,
on overage, more than three time larger thanprevious results. WTe$_2$ films
thicker than 5nm have been successfully synthesized and exhibit theexpected
Td-phase atomic structure. We rationalize epitaxial growth of Td-WTe$_2$ and
propose asimple model to estimate the mean flake size as a function of growth
parameters that can be appliedto other transition metal dichalcogenides
(TMDCs). Based on nucleation theory and Kolmogorov-Johnson-Meh-Avrami (KJMA)
equation, our analytical model supports experimental data showinga critical
coverage of 0.13ML above which WTe$_2$ nucleation becomes negligible. The
quality ofmonolayer WTe$_2$ films is demonstrated from electronic band
structure analysis using angle-resolved photoemission spectroscopy (ARPES) in
agreement with first-principle calculationsperformed on free-standing WTe$_2$
and previous reports.
