There is no universal rule that defines the size you should use your videos. The decision has to come from you with reference in the use of the video. For example: I recently did 4 animations for a college with HYPE. It was an online campaign about some courses. The client informed me that it would be for facebook and whatsapp and that it could be with smaller measures to not get great. (1920 x 1080 full HD, I could make a 780 x 420). Here is the main point ... it has happened to me several times to do in smaller sizes as it comes in the briefing and then the client asks for it or complains that it is not with good resolution, why he decided to put on a television full HD. Based on this, I always make my animations in high, full HD or more and dpeois reduce the size with a good program that does this. (Adobe Premier). And so I do very well. I hope I have helped.
Sorry for my english, I do not speak very well and I do not write.