There are some GTS lora available that could help but there is no way to control the character scale unfortunately. Most of the time, I just generate the character with a neutral background, copy that character onto a background I wanted, then rescale the character at the size you need, refine the whole image and that's pretty much it. It may required a lot of manual editing but you can achieve really great result that way.
Here an example of a miniGTS I made a few months ago.
