I really don't know what is meant by the size of the textbox. If it was any smaller, you wouldn't be able to read the story. If it's in the way during some scenes and you want to see the full image just hit the H button to make the textbox disappear and reappear.
The sex scene duration I can look into but it really wouldn't be anything more than just a couple more lines here and there I would think. I'm very limited in what I can do with the art so it's not like we can do additional positions to increase the duration of the scene.
As for sprite movement, it's part of the limitations to the art. I'm making a game that's inspired by the Japanese VNs and as much as they do additional poses to mix up the art a bit, it's still static images for long periods of time. I just don't have the luxury to do additional poses.