I wanted to make some gaming posters in the style of GTA what are the rules on showing things like guns? or illegal activity and how unique would a character have to be to avoid copyright

just getting started designing my first poster today and got to the point of exporting off canva, and struggling to figure out what px size to use i have googled it but has said different px sizes for different A sizes which i thought would be the same?

is it a good idea to only offer A sizes to keep it simple?