FCPX proxies are 1/2 the linear resolution or 1/4 the pixel resolution of the original material. So for UHD 4k (3840 x 2160) the proxies are 1920 x 1080. For DCI 4k (4096 x 2160) the proxies would be 2048 x 1080. For 6K proxies are likewise 1/2 the linear resolution in each axis.
By "edited a short with HD proxies", this could mean one of several things:
(1) You edited using FCPX-generated proxies and have access to the source material. In that case change the viewer from proxy to optimized/original and export.
(2) You edited using FCPX-generated "external" proxies and don't have the source material. You will need to acquire this from whoever has it. Unfortunately FCPX will not reliably re-link if the proxies are externally placed, so whoever created those must tell you what drive volume name and pathname to the source, and you must configure your system likewise. In that case relink will not be necessary.
In some cases of missing proxies you can ensure the proxies and source media are in the same relation locations as the machine they were generated, switch to proxy mode, you'll see red "missing" icons and you can select those, right-click and pick transcode to proxy, and FCPX will simply update the pointers. Always try this on one or two files first, don't do hundreds of files else it may try to generated new proxies for all those.
(3) You edited using non-FCPX proxies, e.g, generated by the camera or some external utility. This is not currently supported by FCPX but some have reported the ability to relink these to original material in certain cases. The proxies must be some variant of ProRes.