The last time I did anything in After Effects was in school 9 years ago. I am not sure what composition size to use. I am doing a video for a client that will be seen on the internet. I want this video to be higher quality than what is needed for the internet. I would like them to be able to use it for other things. Only I do not know what else they might use it for. I just want to deliver to them a high quality product. This is actually an animated video that they will use for a New Years card to their clients.
If it's going online and just made of graphics, you could make it 4K (3840x2160), 29.97 fps. UHD, as some call it, is becoming more common on YouTube now and a lot of new monitors or computers (such as an iMac) are more than capable of viewing it at that resolution. You could always down-convert for other purposes if necessary.