Although this involves a bit of shameless advertising, FxFactory actually has a plug-in called Add Margin which is designed to get around this limitation in Motion.
Search for “Add Margin” in this page:
https://www.noiseindustries.com/fxfactory/motionpack
The immediate advantage is that it lets you work around the limitation in Motion so that your glows can “bleed” outside the original frame size.
The downside is that the FxPlug feature that allows third-party plug-ins to resize their output does not work in Compressor or when the Motion project is directly embedded in an FCP timeline. This is true for all third-party plug-ins, unfortunately. In other words, you will have to render out your timeline from Motion (for some, this is a serious limitation).
The other method (turning on the “Fixed Resolution” in the Layer/Group tab) works great, it will render correctly outside Motion, but it may require some extra parameter setup depending on which other plug-ins you’re applying down the chain. If you’ve already keyframed point parameters, for instance, you may have to tweak your animation as a result of the image size being different.
Hope this helps,
Gabe
https://www.noiseindustries.com