Of course it’s possible to create custom characters, if you want to adjust sizes and shapes on a character you can switch between pre-made parts or you can use something like Morph Targets, like what Mass Effect does for the face. The game Black Desert does the same type of thing but it allows customization on the entire character, but it’s still morphing the character model.
The issue with large things is really an open-world problem–when things get far away from the origin, the coordinates become a large number and the system is designed to work with variables that take up less memory so when it gets too high you end up loosing precision for physics which breaks the game in many ways. For a skyscraper though, that’s not a problem since that will fit well within the normal space.
Knowing coding will help when doing Blueprints since the logic is the same and you’ll have an idea of what things to look for.
Blender is free, but the issue is that if you want to work in the industry the vast majority of studios use 3ds Max or Maya, and it’s important that you are able to integrate with the studio’s workflow.