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Coronal mass ejections (CMEs) are large-scale eruptions with a typical radialsize at 1 au of 0.21 au but their angular width in interplanetary space isstill mostly unknown, especially for the magnetic ejecta (ME) part of the CME.We take advantage of STEREO-A angular separation of 20$^\circ$-60$^\circ$ fromthe Sun-Earth line from October 2020 to August 2022, and perform a two-partstudy to constrain the angular width of MEs in the ecliptic plane: a) we studyall CMEs that are observed remotely to propagate between the Sun-STEREO-A andthe Sun-Earth lines and determine how many impact one or both spacecraft insitu, and b) we investigate all in situ measurements at STEREO-A or at L1 ofCMEs during the same time period to quantify how many are measured by the twospacecraft. A key finding is that, out of 21 CMEs propagating within 30$^\circ$of either spacecraft, only four impacted both spacecraft and none providedclean magnetic cloud-like signatures at both spacecraft. Combining the twoapproaches, 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 andconsistent with a 2:1 elliptical cross-section of an ellipsoidal ME. We discussthe consequences of this finding for future multi-spacecraft mission designsand for the coherence of CMEs.

The Width of Magnetic Ejecta Measured Near 1 au: Lessons from STEREO-A Measurements in 2021--2022