Image dimensions problems

This theme is just what I was looking for a long time.
But still I have some annoying issues with the images and I think this is the most disappointing aspect of this theme.

1. Due to space I can’t afford to upload pictures larger than 800*445 on my site. In fact, I prefer to be smaller, around 300×250. But if I put them as featured images the slider looks awful. Is there a way to automatically resize/increase if needed the picture in the slider? My latest gabfire theme I used did this thing instantly no matter what picture I’ve uploaded.

2. How can I make the slider stable to a certain dimension? I don’t want it to change depending of the picture size. Same question for the categories.
If the pictures are less than 800×445, the front page, categories page and posts page looks soo bad. And it’s really annoying to manually change every single picture to 800×445. It’s time consuming… and not all of my pictures are so big so I can crop them correctly.

Is there a solution for this?

Maybe the creator of this theme could modify those things.

Thank you.