  Coronal mass ejections (CMEs) are large-scale eruptions with a typical radial
size at 1 au of 0.21 au but their angular width in interplanetary space is
still mostly unknown, especially for the magnetic ejecta (ME) part of the CME.
We take advantage of STEREO-A angular separation of 20$^\circ$-60$^\circ$ from
the Sun-Earth line from October 2020 to August 2022, and perform a two-part
study to constrain the angular width of MEs in the ecliptic plane: a) we study
all CMEs that are observed remotely to propagate between the Sun-STEREO-A and
the Sun-Earth lines and determine how many impact one or both spacecraft in
situ, and b) we investigate all in situ measurements at STEREO-A or at L1 of
CMEs during the same time period to quantify how many are measured by the two
spacecraft. A key finding is that, out of 21 CMEs propagating within 30$^\circ$
of either spacecraft, only four impacted both spacecraft and none provided
clean magnetic cloud-like signatures at both spacecraft. Combining the two
approaches, we conclude that the typical angular width of a ME at 1 au is
$\sim$ 20$^\circ$-30$^\circ$, or 2-3 times less than often assumed and
consistent with a 2:1 elliptical cross-section of an ellipsoidal ME. We discuss
the consequences of this finding for future multi-spacecraft mission designs
and for the coherence of CMEs.
