Social media image sizes change from time to time. Here is a nice visual outlining the most recent dimensions for top social media sites.
There are a lot of recommended post schedules out there, but few take into account a large amount of data across multiple sources. This infographic from SproutSocial shows the general consensus when it comes to posting on Instagram. Bottom-line, the data shows there is more engagement on weekdays (M-F) and during off-business hours (early am and later pm).