Hello. I am Resizer, Ze Robot.

You can use the form on the image page, so you can resize to any meaningful resolution

I could, but quality would be awful. There are plans to use supersampling to enlarge pictures slightly (like 10-20%), but for now, I do not resize to higher resolutions than source image.

There are 2 sources of images to resize:
1.subreddit added to watchlist. If you think that I can help on some subreddit - please pm me. All posts with about 10 upvotes added to resize queue. (Upvote check needed to counter shitposters, we all dislike them, right?)
2.username mention in comments. If you see cool picture on any subreddit, and want it resized - just comment something with /u/ze-robot, I'll check my inbox and will answer to your comment.

I crop image keeping the central part. Sometimes it gives funny result, but overall it is the good way.