  (Abridged) We study the polarisation properties, magnetic field strength, and
synchrotron emission scale-height of Milky-Way-like galaxies in comparison with
other spiral galaxies. We use our 3D-emission model of the Milky Way Galaxy for
viewing the Milky Way from outside at various inclinations as spiral galaxies
are observed. When seen edge-on the synchrotron emission from the Milky Way has
an exponential scale-height of about 0.74 kpc, which is much smaller than the
values obtained from previous models. We find that current analysis methods
overestimate the scale-height of synchrotron emission of galaxies by about 10%
at an inclination of 80 degree and about 40% at an inclination of 70 degree
because of contamination from the disk. The observed RMs for face-on galaxies
derived from high-frequency polarisation measurements approximate to the
Faraday depths (FDs) when scaled by a factor of two. For edge-on galaxies, the
observed RMs are indicative of the orientation of the large-scale magnetic
field, but are not well related with the FDs. Assuming energy equipartition
between the magnetic field and particles for the Milky Way results in an
average magnetic-field strength, which is about two times larger than the
intrinsic value for a K factor of 100. The number distribution of the
integrated polarisation percentages of a large sample of unresolved
Milky-Way-like galaxies peaks at about 4.2% at 4.8 GHz and at about 0.8% at
1.4GHz. Integrated polarisation angles rotated by 90 degree align very well
with the position angles of the major axes, implying that unresolved galaxies
do not have intrinsic RMs.
