The standard companion banner size for TrueView in-stream ads on YouTube is:
300×60 is a standard companion size for TrueView ads on YouTube.
Companion banners are optional image banner ads that appear on the top right corner of the watch page for the duration of the content video. When an in-stream video ad ends (or is skipped) the companion banner remains on the YouTube page and the user can click on it at any time. If you choose to include companion banners, be aware that that they’ll display only in the context of a desktop watch page on YouTube. They do not appear on mobile devices, embedded players, connected TVs, or on game consoles.
In some cases, if a custom companion creative (300×60) is not supplied and the ad is a public YouTube hosted video, a video wall companion will be automatically generated which includes information about the channel hosting the video ad.
Read more here: https://support.google.com/displayspecs/answer/6244557?hl=en